mm/thp: fix __split_huge_pmd_locked() on shmem migration entry

Patch series "mm/thp: fix THP splitting unmap BUGs and related", v10.

Here is v2 batch of long-standing THP bug fixes that I had not got
around to sending before, but prompted now by Wang Yugui's report
https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/

Wang Yugui has tested a rollup of these fixes applied to 5.10.39, and
they have done no harm, but have *not* fixed that issue: something more
is needed and I have no idea of what.

This patch (of 7):

Stressing huge tmpfs page migration racing hole punch often crashed on
the VM_BUG_ON(!pmd_present) in pmdp_huge_clear_flush(), with DEBUG_VM=y
kernel; or shortly afterwards, on a bad dereference in
__split_huge_pmd_locked() when DEBUG_VM=n.  They forgot to allow for pmd
migration entries in the non-anonymous case.

Full disclosure: those particular experiments were on a kernel with more
relaxed mmap_lock and i_mmap_rwsem locking, and were not repeated on the
vanilla kernel: it is conceivable that stricter locking happens to avoid
those cases, or makes them less likely; but __split_huge_pmd_locked()
already allowed for pmd migration entries when handling anonymous THPs,
so this commit brings the shmem and file THP handling into line.

And while there: use old_pmd rather than _pmd, as in the following
blocks; and make it clearer to the eye that the !vma_is_anonymous()
block is self-contained, making an early return after accounting for
unmapping.

Link: https://lkml.kernel.org/r/af88612-1473-2eaa-903-8d1a448b26@google.com
Link: https://lkml.kernel.org/r/dd221a99-efb3-cd1d-6256-7e646af29314@google.com
Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Wang Yugui <wangyugui@e16-tech.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Jue Wang <juew@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 63ed6b2..42cfefc 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2044,7 +2044,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
        count_vm_event(THP_SPLIT_PMD);
 
        if (!vma_is_anonymous(vma)) {
-               _pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
+               old_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
                /*
                 * We are going to unmap this huge page. So
                 * just go ahead and zap it
@@ -2053,16 +2053,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
                        zap_deposited_table(mm, pmd);
                if (vma_is_special_huge(vma))
                        return;
-               page = pmd_page(_pmd);
-               if (!PageDirty(page) && pmd_dirty(_pmd))
-                       set_page_dirty(page);
-               if (!PageReferenced(page) && pmd_young(_pmd))
-                       SetPageReferenced(page);
-               page_remove_rmap(page, true);
-               put_page(page);
+               if (unlikely(is_pmd_migration_entry(old_pmd))) {
+                       swp_entry_t entry;
+
+                       entry = pmd_to_swp_entry(old_pmd);
+                       page = migration_entry_to_page(entry);
+               } else {
+                       page = pmd_page(old_pmd);
+                       if (!PageDirty(page) && pmd_dirty(old_pmd))
+                               set_page_dirty(page);
+                       if (!PageReferenced(page) && pmd_young(old_pmd))
+                               SetPageReferenced(page);
+                       page_remove_rmap(page, true);
+                       put_page(page);
+               }
                add_mm_counter(mm, mm_counter_file(page), -HPAGE_PMD_NR);
                return;
-       } else if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
+       }
+
+       if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
                /*
                 * FIXME: Do we want to invalidate secondary mmu by calling
                 * mmu_notifier_invalidate_range() see comments below inside

diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index c2210e1..4e640ba 100644 (file)
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -135,9 +135,8 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
 {
        pmd_t pmd;
        VM_BUG_ON(address & ~HPAGE_PMD_MASK);
-       VM_BUG_ON(!pmd_present(*pmdp));
-       /* Below assumes pmd_present() is true */
-       VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
+       VM_BUG_ON(pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
+                          !pmd_devmap(*pmdp));
        pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
        flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
        return pmd;