mm/hmm.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * Copyright 2013 Red Hat Inc.
   4  *
   5  * Authors: Jérôme Glisse <jglisse@redhat.com>
   6  */
   7 /*
   8  * Refer to include/linux/hmm.h for information about heterogeneous memory
   9  * management or HMM for short.
  10  */
  11 #include <linux/pagewalk.h>
  12 #include <linux/hmm.h>
  13 #include <linux/init.h>
  14 #include <linux/rmap.h>
  15 #include <linux/swap.h>
  16 #include <linux/slab.h>
  17 #include <linux/sched.h>
  18 #include <linux/mmzone.h>
  19 #include <linux/pagemap.h>
  20 #include <linux/swapops.h>
  21 #include <linux/hugetlb.h>
  22 #include <linux/memremap.h>
  23 #include <linux/sched/mm.h>
  24 #include <linux/jump_label.h>
  25 #include <linux/dma-mapping.h>
  26 #include <linux/mmu_notifier.h>
  27 #include <linux/memory_hotplug.h>
  28
  29 struct hmm_vma_walk {
  30         struct hmm_range        *range;
  31         unsigned long           last;
  32 };
  33
  34 enum {
  35         HMM_NEED_FAULT = 1 << 0,
  36         HMM_NEED_WRITE_FAULT = 1 << 1,
  37         HMM_NEED_ALL_BITS = HMM_NEED_FAULT | HMM_NEED_WRITE_FAULT,
  38 };
  39
  40 static int hmm_pfns_fill(unsigned long addr, unsigned long end,
  41                          struct hmm_range *range, unsigned long cpu_flags)
  42 {
  43         unsigned long i = (addr - range->start) >> PAGE_SHIFT;
  44
  45         for (; addr < end; addr += PAGE_SIZE, i++)
  46                 range->hmm_pfns[i] = cpu_flags;
  47         return 0;
  48 }
  49
  50 /*
  51  * hmm_vma_fault() - fault in a range lacking valid pmd or pte(s)
  52  * @addr: range virtual start address (inclusive)
  53  * @end: range virtual end address (exclusive)
  54  * @required_fault: HMM_NEED_* flags
  55  * @walk: mm_walk structure
  56  * Return: -EBUSY after page fault, or page fault error
  57  *
  58  * This function will be called whenever pmd_none() or pte_none() returns true,
  59  * or whenever there is no page directory covering the virtual address range.
  60  */
  61 static int hmm_vma_fault(unsigned long addr, unsigned long end,
  62                          unsigned int required_fault, struct mm_walk *walk)
  63 {
  64         struct hmm_vma_walk *hmm_vma_walk = walk->private;
  65         struct vm_area_struct *vma = walk->vma;
  66         unsigned int fault_flags = FAULT_FLAG_REMOTE;
  67
  68         WARN_ON_ONCE(!required_fault);
  69         hmm_vma_walk->last = addr;
  70
  71         if (required_fault & HMM_NEED_WRITE_FAULT) {
  72                 if (!(vma->vm_flags & VM_WRITE))
  73                         return -EPERM;
  74                 fault_flags |= FAULT_FLAG_WRITE;
  75         }
  76
  77         for (; addr < end; addr += PAGE_SIZE)
  78                 if (handle_mm_fault(vma, addr, fault_flags) & VM_FAULT_ERROR)
  79                         return -EFAULT;
  80         return -EBUSY;
  81 }
  82
  83 static unsigned int hmm_pte_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
  84                                        unsigned long pfn_req_flags,
  85                                        unsigned long cpu_flags)
  86 {
  87         struct hmm_range *range = hmm_vma_walk->range;
  88
  89         /*
  90          * So we not only consider the individual per page request we also
  91          * consider the default flags requested for the range. The API can
  92          * be used 2 ways. The first one where the HMM user coalesces
  93          * multiple page faults into one request and sets flags per pfn for
  94          * those faults. The second one where the HMM user wants to pre-
  95          * fault a range with specific flags. For the latter one it is a
  96          * waste to have the user pre-fill the pfn arrays with a default
  97          * flags value.
  98          */
  99         pfn_req_flags &= range->pfn_flags_mask;
 100         pfn_req_flags |= range->default_flags;
 101
 102         /* We aren't ask to do anything ... */
 103         if (!(pfn_req_flags & HMM_PFN_REQ_FAULT))
 104                 return 0;
 105
 106         /* Need to write fault ? */
 107         if ((pfn_req_flags & HMM_PFN_REQ_WRITE) &&
 108             !(cpu_flags & HMM_PFN_WRITE))
 109                 return HMM_NEED_FAULT | HMM_NEED_WRITE_FAULT;
 110
 111         /* If CPU page table is not valid then we need to fault */
 112         if (!(cpu_flags & HMM_PFN_VALID))
 113                 return HMM_NEED_FAULT;
 114         return 0;
 115 }
 116
 117 static unsigned int
 118 hmm_range_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
 119                      const unsigned long hmm_pfns[], unsigned long npages,
 120                      unsigned long cpu_flags)
 121 {
 122         struct hmm_range *range = hmm_vma_walk->range;
 123         unsigned int required_fault = 0;
 124         unsigned long i;
 125
 126         /*
 127          * If the default flags do not request to fault pages, and the mask does
 128          * not allow for individual pages to be faulted, then
 129          * hmm_pte_need_fault() will always return 0.
 130          */
 131         if (!((range->default_flags | range->pfn_flags_mask) &
 132               HMM_PFN_REQ_FAULT))
 133                 return 0;
 134
 135         for (i = 0; i < npages; ++i) {
 136                 required_fault |= hmm_pte_need_fault(hmm_vma_walk, hmm_pfns[i],
 137                                                      cpu_flags);
 138                 if (required_fault == HMM_NEED_ALL_BITS)
 139                         return required_fault;
 140         }
 141         return required_fault;
 142 }
 143
 144 static int hmm_vma_walk_hole(unsigned long addr, unsigned long end,
 145                              __always_unused int depth, struct mm_walk *walk)
 146 {
 147         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 148         struct hmm_range *range = hmm_vma_walk->range;
 149         unsigned int required_fault;
 150         unsigned long i, npages;
 151         unsigned long *hmm_pfns;
 152
 153         i = (addr - range->start) >> PAGE_SHIFT;
 154         npages = (end - addr) >> PAGE_SHIFT;
 155         hmm_pfns = &range->hmm_pfns[i];
 156         required_fault =
 157                 hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0);
 158         if (!walk->vma) {
 159                 if (required_fault)
 160                         return -EFAULT;
 161                 return hmm_pfns_fill(addr, end, range, HMM_PFN_ERROR);
 162         }
 163         if (required_fault)
 164                 return hmm_vma_fault(addr, end, required_fault, walk);
 165         return hmm_pfns_fill(addr, end, range, 0);
 166 }
 167
 168 static inline unsigned long hmm_pfn_flags_order(unsigned long order)
 169 {
 170         return order << HMM_PFN_ORDER_SHIFT;
 171 }
 172
 173 static inline unsigned long pmd_to_hmm_pfn_flags(struct hmm_range *range,
 174                                                  pmd_t pmd)
 175 {
 176         if (pmd_protnone(pmd))
 177                 return 0;
 178         return (pmd_write(pmd) ? (HMM_PFN_VALID | HMM_PFN_WRITE) :
 179                                  HMM_PFN_VALID) |
 180                hmm_pfn_flags_order(PMD_SHIFT - PAGE_SHIFT);
 181 }
 182
 183 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 184 static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 185                               unsigned long end, unsigned long hmm_pfns[],
 186                               pmd_t pmd)
 187 {
 188         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 189         struct hmm_range *range = hmm_vma_walk->range;
 190         unsigned long pfn, npages, i;
 191         unsigned int required_fault;
 192         unsigned long cpu_flags;
 193
 194         npages = (end - addr) >> PAGE_SHIFT;
 195         cpu_flags = pmd_to_hmm_pfn_flags(range, pmd);
 196         required_fault =
 197                 hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, cpu_flags);
 198         if (required_fault)
 199                 return hmm_vma_fault(addr, end, required_fault, walk);
 200
 201         pfn = pmd_pfn(pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 202         for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++)
 203                 hmm_pfns[i] = pfn | cpu_flags;
 204         return 0;
 205 }
 206 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
 207 /* stub to allow the code below to compile */
 208 int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 209                 unsigned long end, unsigned long hmm_pfns[], pmd_t pmd);
 210 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 211
 212 static inline bool hmm_is_device_private_entry(struct hmm_range *range,
 213                 swp_entry_t entry)
 214 {
 215         return is_device_private_entry(entry) &&
 216                 device_private_entry_to_page(entry)->pgmap->owner ==
 217                 range->dev_private_owner;
 218 }
 219
 220 static inline unsigned long pte_to_hmm_pfn_flags(struct hmm_range *range,
 221                                                  pte_t pte)
 222 {
 223         if (pte_none(pte) || !pte_present(pte) || pte_protnone(pte))
 224                 return 0;
 225         return pte_write(pte) ? (HMM_PFN_VALID | HMM_PFN_WRITE) : HMM_PFN_VALID;
 226 }
 227
 228 static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 229                               unsigned long end, pmd_t *pmdp, pte_t *ptep,
 230                               unsigned long *hmm_pfn)
 231 {
 232         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 233         struct hmm_range *range = hmm_vma_walk->range;
 234         unsigned int required_fault;
 235         unsigned long cpu_flags;
 236         pte_t pte = *ptep;
 237         uint64_t pfn_req_flags = *hmm_pfn;
 238
 239         if (pte_none(pte)) {
 240                 required_fault =
 241                         hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0);
 242                 if (required_fault)
 243                         goto fault;
 244                 *hmm_pfn = 0;
 245                 return 0;
 246         }
 247
 248         if (!pte_present(pte)) {
 249                 swp_entry_t entry = pte_to_swp_entry(pte);
 250
 251                 /*
 252                  * Never fault in device private pages pages, but just report
 253                  * the PFN even if not present.
 254                  */
 255                 if (hmm_is_device_private_entry(range, entry)) {
 256                         cpu_flags = HMM_PFN_VALID;
 257                         if (is_write_device_private_entry(entry))
 258                                 cpu_flags |= HMM_PFN_WRITE;
 259                         *hmm_pfn = device_private_entry_to_pfn(entry) |
 260                                         cpu_flags;
 261                         return 0;
 262                 }
 263
 264                 required_fault =
 265                         hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0);
 266                 if (!required_fault) {
 267                         *hmm_pfn = 0;
 268                         return 0;
 269                 }
 270
 271                 if (!non_swap_entry(entry))
 272                         goto fault;
 273
 274                 if (is_migration_entry(entry)) {
 275                         pte_unmap(ptep);
 276                         hmm_vma_walk->last = addr;
 277                         migration_entry_wait(walk->mm, pmdp, addr);
 278                         return -EBUSY;
 279                 }
 280
 281                 /* Report error for everything else */
 282                 pte_unmap(ptep);
 283                 return -EFAULT;
 284         }
 285
 286         cpu_flags = pte_to_hmm_pfn_flags(range, pte);
 287         required_fault =
 288                 hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags);
 289         if (required_fault)
 290                 goto fault;
 291
 292         /*
 293          * Since each architecture defines a struct page for the zero page, just
 294          * fall through and treat it like a normal page.
 295          */
 296         if (pte_special(pte) && !is_zero_pfn(pte_pfn(pte))) {
 297                 if (hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0)) {
 298                         pte_unmap(ptep);
 299                         return -EFAULT;
 300                 }
 301                 *hmm_pfn = HMM_PFN_ERROR;
 302                 return 0;
 303         }
 304
 305         *hmm_pfn = pte_pfn(pte) | cpu_flags;
 306         return 0;
 307
 308 fault:
 309         pte_unmap(ptep);
 310         /* Fault any virtual address we were asked to fault */
 311         return hmm_vma_fault(addr, end, required_fault, walk);
 312 }
 313
 314 static int hmm_vma_walk_pmd(pmd_t *pmdp,
 315                             unsigned long start,
 316                             unsigned long end,
 317                             struct mm_walk *walk)
 318 {
 319         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 320         struct hmm_range *range = hmm_vma_walk->range;
 321         unsigned long *hmm_pfns =
 322                 &range->hmm_pfns[(start - range->start) >> PAGE_SHIFT];
 323         unsigned long npages = (end - start) >> PAGE_SHIFT;
 324         unsigned long addr = start;
 325         pte_t *ptep;
 326         pmd_t pmd;
 327
 328 again:
 329         pmd = READ_ONCE(*pmdp);
 330         if (pmd_none(pmd))
 331                 return hmm_vma_walk_hole(start, end, -1, walk);
 332
 333         if (thp_migration_supported() && is_pmd_migration_entry(pmd)) {
 334                 if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0)) {
 335                         hmm_vma_walk->last = addr;
 336                         pmd_migration_entry_wait(walk->mm, pmdp);
 337                         return -EBUSY;
 338                 }
 339                 return hmm_pfns_fill(start, end, range, 0);
 340         }
 341
 342         if (!pmd_present(pmd)) {
 343                 if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
 344                         return -EFAULT;
 345                 return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
 346         }
 347
 348         if (pmd_devmap(pmd) || pmd_trans_huge(pmd)) {
 349                 /*
 350                  * No need to take pmd_lock here, even if some other thread
 351                  * is splitting the huge pmd we will get that event through
 352                  * mmu_notifier callback.
 353                  *
 354                  * So just read pmd value and check again it's a transparent
 355                  * huge or device mapping one and compute corresponding pfn
 356                  * values.
 357                  */
 358                 pmd = pmd_read_atomic(pmdp);
 359                 barrier();
 360                 if (!pmd_devmap(pmd) && !pmd_trans_huge(pmd))
 361                         goto again;
 362
 363                 return hmm_vma_handle_pmd(walk, addr, end, hmm_pfns, pmd);
 364         }
 365
 366         /*
 367          * We have handled all the valid cases above ie either none, migration,
 368          * huge or transparent huge. At this point either it is a valid pmd
 369          * entry pointing to pte directory or it is a bad pmd that will not
 370          * recover.
 371          */
 372         if (pmd_bad(pmd)) {
 373                 if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
 374                         return -EFAULT;
 375                 return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
 376         }
 377
 378         ptep = pte_offset_map(pmdp, addr);
 379         for (; addr < end; addr += PAGE_SIZE, ptep++, hmm_pfns++) {
 380                 int r;
 381
 382                 r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, hmm_pfns);
 383                 if (r) {
 384                         /* hmm_vma_handle_pte() did pte_unmap() */
 385                         return r;
 386                 }
 387         }
 388         pte_unmap(ptep - 1);
 389         return 0;
 390 }
 391
 392 #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && \
 393     defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
 394 static inline unsigned long pud_to_hmm_pfn_flags(struct hmm_range *range,
 395                                                  pud_t pud)
 396 {
 397         if (!pud_present(pud))
 398                 return 0;
 399         return (pud_write(pud) ? (HMM_PFN_VALID | HMM_PFN_WRITE) :
 400                                  HMM_PFN_VALID) |
 401                hmm_pfn_flags_order(PUD_SHIFT - PAGE_SHIFT);
 402 }
 403
 404 static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
 405                 struct mm_walk *walk)
 406 {
 407         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 408         struct hmm_range *range = hmm_vma_walk->range;
 409         unsigned long addr = start;
 410         pud_t pud;
 411         int ret = 0;
 412         spinlock_t *ptl = pud_trans_huge_lock(pudp, walk->vma);
 413
 414         if (!ptl)
 415                 return 0;
 416
 417         /* Normally we don't want to split the huge page */
 418         walk->action = ACTION_CONTINUE;
 419
 420         pud = READ_ONCE(*pudp);
 421         if (pud_none(pud)) {
 422                 spin_unlock(ptl);
 423                 return hmm_vma_walk_hole(start, end, -1, walk);
 424         }
 425
 426         if (pud_huge(pud) && pud_devmap(pud)) {
 427                 unsigned long i, npages, pfn;
 428                 unsigned int required_fault;
 429                 unsigned long *hmm_pfns;
 430                 unsigned long cpu_flags;
 431
 432                 if (!pud_present(pud)) {
 433                         spin_unlock(ptl);
 434                         return hmm_vma_walk_hole(start, end, -1, walk);
 435                 }
 436
 437                 i = (addr - range->start) >> PAGE_SHIFT;
 438                 npages = (end - addr) >> PAGE_SHIFT;
 439                 hmm_pfns = &range->hmm_pfns[i];
 440
 441                 cpu_flags = pud_to_hmm_pfn_flags(range, pud);
 442                 required_fault = hmm_range_need_fault(hmm_vma_walk, hmm_pfns,
 443                                                       npages, cpu_flags);
 444                 if (required_fault) {
 445                         spin_unlock(ptl);
 446                         return hmm_vma_fault(addr, end, required_fault, walk);
 447                 }
 448
 449                 pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 450                 for (i = 0; i < npages; ++i, ++pfn)
 451                         hmm_pfns[i] = pfn | cpu_flags;
 452                 goto out_unlock;
 453         }
 454
 455         /* Ask for the PUD to be split */
 456         walk->action = ACTION_SUBTREE;
 457
 458 out_unlock:
 459         spin_unlock(ptl);
 460         return ret;
 461 }
 462 #else
 463 #define hmm_vma_walk_pud        NULL
 464 #endif
 465
 466 #ifdef CONFIG_HUGETLB_PAGE
 467 static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
 468                                       unsigned long start, unsigned long end,
 469                                       struct mm_walk *walk)
 470 {
 471         unsigned long addr = start, i, pfn;
 472         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 473         struct hmm_range *range = hmm_vma_walk->range;
 474         struct vm_area_struct *vma = walk->vma;
 475         unsigned int required_fault;
 476         unsigned long pfn_req_flags;
 477         unsigned long cpu_flags;
 478         spinlock_t *ptl;
 479         pte_t entry;
 480
 481         ptl = huge_pte_lock(hstate_vma(vma), walk->mm, pte);
 482         entry = huge_ptep_get(pte);
 483
 484         i = (start - range->start) >> PAGE_SHIFT;
 485         pfn_req_flags = range->hmm_pfns[i];
 486         cpu_flags = pte_to_hmm_pfn_flags(range, entry) |
 487                     hmm_pfn_flags_order(huge_page_order(hstate_vma(vma)));
 488         required_fault =
 489                 hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags);
 490         if (required_fault) {
 491                 spin_unlock(ptl);
 492                 return hmm_vma_fault(addr, end, required_fault, walk);
 493         }
 494
 495         pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT);
 496         for (; addr < end; addr += PAGE_SIZE, i++, pfn++)
 497                 range->hmm_pfns[i] = pfn | cpu_flags;
 498
 499         spin_unlock(ptl);
 500         return 0;
 501 }
 502 #else
 503 #define hmm_vma_walk_hugetlb_entry NULL
 504 #endif /* CONFIG_HUGETLB_PAGE */
 505
 506 static int hmm_vma_walk_test(unsigned long start, unsigned long end,
 507                              struct mm_walk *walk)
 508 {
 509         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 510         struct hmm_range *range = hmm_vma_walk->range;
 511         struct vm_area_struct *vma = walk->vma;
 512
 513         if (!(vma->vm_flags & (VM_IO | VM_PFNMAP | VM_MIXEDMAP)) &&
 514             vma->vm_flags & VM_READ)
 515                 return 0;
 516
 517         /*
 518          * vma ranges that don't have struct page backing them or map I/O
 519          * devices directly cannot be handled by hmm_range_fault().
 520          *
 521          * If the vma does not allow read access, then assume that it does not
 522          * allow write access either. HMM does not support architectures that
 523          * allow write without read.
 524          *
 525          * If a fault is requested for an unsupported range then it is a hard
 526          * failure.
 527          */
 528         if (hmm_range_need_fault(hmm_vma_walk,
 529                                  range->hmm_pfns +
 530                                          ((start - range->start) >> PAGE_SHIFT),
 531                                  (end - start) >> PAGE_SHIFT, 0))
 532                 return -EFAULT;
 533
 534         hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
 535
 536         /* Skip this vma and continue processing the next vma. */
 537         return 1;
 538 }
 539
 540 static const struct mm_walk_ops hmm_walk_ops = {
 541         .pud_entry      = hmm_vma_walk_pud,
 542         .pmd_entry      = hmm_vma_walk_pmd,
 543         .pte_hole       = hmm_vma_walk_hole,
 544         .hugetlb_entry  = hmm_vma_walk_hugetlb_entry,
 545         .test_walk      = hmm_vma_walk_test,
 546 };
 547
 548 /**
 549  * hmm_range_fault - try to fault some address in a virtual address range
 550  * @range:      argument structure
 551  *
 552  * Returns 0 on success or one of the following error codes:
 553  *
 554  * -EINVAL:     Invalid arguments or mm or virtual address is in an invalid vma
 555  *              (e.g., device file vma).
 556  * -ENOMEM:     Out of memory.
 557  * -EPERM:      Invalid permission (e.g., asking for write and range is read
 558  *              only).
 559  * -EBUSY:      The range has been invalidated and the caller needs to wait for
 560  *              the invalidation to finish.
 561  * -EFAULT:     A page was requested to be valid and could not be made valid
 562  *              ie it has no backing VMA or it is illegal to access
 563  *
 564  * This is similar to get_user_pages(), except that it can read the page tables
 565  * without mutating them (ie causing faults).
 566  */
 567 int hmm_range_fault(struct hmm_range *range)
 568 {
 569         struct hmm_vma_walk hmm_vma_walk = {
 570                 .range = range,
 571                 .last = range->start,
 572         };
 573         struct mm_struct *mm = range->notifier->mm;
 574         int ret;
 575
 576         mmap_assert_locked(mm);
 577
 578         do {
 579                 /* If range is no longer valid force retry. */
 580                 if (mmu_interval_check_retry(range->notifier,
 581                                              range->notifier_seq))
 582                         return -EBUSY;
 583                 ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
 584                                       &hmm_walk_ops, &hmm_vma_walk);
 585                 /*
 586                  * When -EBUSY is returned the loop restarts with
 587                  * hmm_vma_walk.last set to an address that has not been stored
 588                  * in pfns. All entries < last in the pfn array are set to their
 589                  * output, and all >= are still at their input values.
 590                  */
 591         } while (ret == -EBUSY);
 592         return ret;
 593 }
 594 EXPORT_SYMBOL(hmm_range_fault);