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mm/gup.c: stricter check on THP migration entry during follow_pmd_mask
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/20211217062559.737063-1-lixinhai.lxh@gmail.com Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com> Reviewed-by: "Huang, Ying" <ying.huang@intel.com> Reviewed-by: Miaohe Lin <linmiaohe@huawei.com> Cc: Zi Yan <ziy@nvidia.com> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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1 changed files with 9 additions and 4 deletions
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mm/gup.c
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mm/gup.c
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@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
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}
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retry:
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if (!pmd_present(pmdval)) {
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/*
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* Should never reach here, if thp migration is not supported;
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* Otherwise, it must be a thp migration entry.
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*/
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VM_BUG_ON(!thp_migration_supported() ||
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!is_pmd_migration_entry(pmdval));
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if (likely(!(flags & FOLL_MIGRATION)))
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return no_page_table(vma, flags);
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VM_BUG_ON(thp_migration_supported() &&
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!is_pmd_migration_entry(pmdval));
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if (is_pmd_migration_entry(pmdval))
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pmd_migration_entry_wait(mm, pmd);
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pmd_migration_entry_wait(mm, pmd);
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pmdval = READ_ONCE(*pmd);
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/*
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* MADV_DONTNEED may convert the pmd to null because
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