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mm/gup.c: stricter check on THP migration entry during follow_pmd_mask

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When BUG_ON check for THP migration entry, the existing code only check
thp_migration_supported case, but not for !thp_migration_supported case.
If !thp_migration_supported() and !pmd_present(), the original code may
dead loop in theory.  To make the BUG_ON check consistent, we need catch
both cases.

Move the BUG_ON check one step earlier, because if the bug happen we
should know it instead of depend on FOLL_MIGRATION been used by caller.

Because pmdval instead of *pmd is read by the is_pmd_migration_entry()
check, the existing code don't help to avoid useless locking within
pmd_migration_entry_wait(), so remove that check.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Li Xinhai <[email protected]>
Reviewed-by: "Huang, Ying" <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Li Xinhai authored and Linus Torvalds committed 2022-01-15 16:30:26 +02:00
1 parent 677b2a8c1f
commit 28b0ee3fb3
1 file changed
+9 -4
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mm/gup.c
+9 -4
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@@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
}
retry:
if (!pmd_present(pmdval)) {
/*
* Should never reach here, if thp migration is not supported;
* Otherwise, it must be a thp migration entry.
*/
VM_BUG_ON(!thp_migration_supported() ||
!is_pmd_migration_entry(pmdval));
if (likely(!(flags & FOLL_MIGRATION)))
return no_page_table(vma, flags);
VM_BUG_ON(thp_migration_supported() &&
!is_pmd_migration_entry(pmdval));
if (is_pmd_migration_entry(pmdval))
pmd_migration_entry_wait(mm, pmd);
pmd_migration_entry_wait(mm, pmd);
pmdval = READ_ONCE(*pmd);
/*
* MADV_DONTNEED may convert the pmd to null because