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mm/huge_memory: comment the subtly logic in __split_huge_pmd

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It's dangerous and wrong to call page_folio(pmd_page(*pmd)) when pmd isn't
present. But the caller guarantees pmd is present when folio is set. So we
should be safe here. Add comment to make it clear.

Link: https://lkml.kernel.org/r/20220704132201.14611-16-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Miaohe Lin 2022-07-04 21:22:00 +08:00 committed by akpm
parent d764afedfb
commit cea3332808
1 changed files with 4 additions and 0 deletions
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4
mm/huge_memory.c
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@ -2235,6 +2235,10 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
is_pmd_migration_entry(*pmd)) {
/*
* It's safe to call pmd_page when folio is set because it's
* guaranteed that pmd is present.
*/
if (folio && folio != page_folio(pmd_page(*pmd)))
goto out;
__split_huge_pmd_locked(vma, pmd, range.start, freeze);