mm/gup: reorganize internal_get_user_pages_fast()

Patch series "Add a seqcount between gup_fast and copy_page_range()", v4.

As discussed and suggested by Linus use a seqcount to close the small race
between gup_fast and copy_page_range().

Ahmed confirms that raw_write_seqcount_begin() is the correct API to use
in this case and it doesn't trigger any lockdeps.

I was able to test it using two threads, one forking and the other using
ibv_reg_mr() to trigger GUP fast.  Modifying copy_page_range() to sleep
made the window large enough to reliably hit to test the logic.

This patch (of 2):

The next patch in this series makes the lockless flow a little more
complex, so move the entire block into a new function and remove a level
of indention.  Tidy a bit of cruft:

 - addr is always the same as start, so use start

 - Use the modern check_add_overflow() for computing end = start + len

 - nr_pinned/pages << PAGE_SHIFT needs the LHS to be unsigned long to
   avoid shift overflow, make the variables unsigned long to avoid coding
   casts in both places. nr_pinned was missing its cast

 - The handling of ret and nr_pinned can be streamlined a bit

No functional change.

Link: https://lkml.kernel.org/r/0-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com
Link: https://lkml.kernel.org/r/1-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index 98eb8e6..c7e2430 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2677,13 +2677,43 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
        return ret;
 }
 
-static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
+static unsigned long lockless_pages_from_mm(unsigned long start,
+                                           unsigned long end,
+                                           unsigned int gup_flags,
+                                           struct page **pages)
+{
+       unsigned long flags;
+       int nr_pinned = 0;
+
+       if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) ||
+           !gup_fast_permitted(start, end))
+               return 0;
+
+       /*
+        * Disable interrupts. The nested form is used, in order to allow full,
+        * general purpose use of this routine.
+        *
+        * With interrupts disabled, we block page table pages from being freed
+        * from under us. See struct mmu_table_batch comments in
+        * include/asm-generic/tlb.h for more details.
+        *
+        * We do not adopt an rcu_read_lock() here as we also want to block IPIs
+        * that come from THPs splitting.
+        */
+       local_irq_save(flags);
+       gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
+       local_irq_restore(flags);
+       return nr_pinned;
+}
+
+static int internal_get_user_pages_fast(unsigned long start,
+                                       unsigned long nr_pages,
                                        unsigned int gup_flags,
                                        struct page **pages)
 {
-       unsigned long addr, len, end;
-       unsigned long flags;
-       int nr_pinned = 0, ret = 0;
+       unsigned long len, end;
+       unsigned long nr_pinned;
+       int ret;
 
        if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
                                       FOLL_FORCE | FOLL_PIN | FOLL_GET |
@@ -2697,54 +2727,33 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
                might_lock_read(&current->mm->mmap_lock);
 
        start = untagged_addr(start) & PAGE_MASK;
-       addr = start;
-       len = (unsigned long) nr_pages << PAGE_SHIFT;
-       end = start + len;
-
-       if (end <= start)
+       len = nr_pages << PAGE_SHIFT;
+       if (check_add_overflow(start, len, &end))
                return 0;
        if (unlikely(!access_ok((void __user *)start, len)))
                return -EFAULT;
 
-       /*
-        * Disable interrupts. The nested form is used, in order to allow
-        * full, general purpose use of this routine.
-        *
-        * With interrupts disabled, we block page table pages from being
-        * freed from under us. See struct mmu_table_batch comments in
-        * include/asm-generic/tlb.h for more details.
-        *
-        * We do not adopt an rcu_read_lock(.) here as we also want to
-        * block IPIs that come from THPs splitting.
-        */
-       if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) {
-               unsigned long fast_flags = gup_flags;
-
-               local_irq_save(flags);
-               gup_pgd_range(addr, end, fast_flags, pages, &nr_pinned);
-               local_irq_restore(flags);
-               ret = nr_pinned;
-       }
-
-       if (nr_pinned < nr_pages && !(gup_flags & FOLL_FAST_ONLY)) {
-               /* Try to get the remaining pages with get_user_pages */
-               start += nr_pinned << PAGE_SHIFT;
-               pages += nr_pinned;
+       nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages);
+       if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
+               return nr_pinned;
 
-               ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned,
-                                             gup_flags, pages);
-
-               /* Have to be a bit careful with return values */
-               if (nr_pinned > 0) {
-                       if (ret < 0)
-                               ret = nr_pinned;
-                       else
-                               ret += nr_pinned;
-               }
+       /* Slow path: try to get the remaining pages with get_user_pages */
+       start += nr_pinned << PAGE_SHIFT;
+       pages += nr_pinned;
+       ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags,
+                                     pages);
+       if (ret < 0) {
+               /*
+                * The caller has to unpin the pages we already pinned so
+                * returning -errno is not an option
+                */
+               if (nr_pinned)
+                       return nr_pinned;
+               return ret;
        }
-
-       return ret;
+       return ret + nr_pinned;
 }
+
 /**
  * get_user_pages_fast_only() - pin user pages in memory
  * @start:      starting user address