Re: [PATCH v3 4/7] mm/thp: Carry over dirty bit when thp splits on pmd

From: Huang, Ying
Date: Wed Aug 10 2022 - 02:24:44 EST


Peter Xu <peterx@xxxxxxxxxx> writes:

> Carry over the dirty bit from pmd to pte when a huge pmd splits. It
> shouldn't be a correctness issue since when pmd_dirty() we'll have the page
> marked dirty anyway, however having dirty bit carried over helps the next
> initial writes of split ptes on some archs like x86.
>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
> mm/huge_memory.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0611b2fd145a..e8e78d1bac5f 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2005,7 +2005,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> pgtable_t pgtable;
> pmd_t old_pmd, _pmd;
> bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
> - bool anon_exclusive = false;
> + bool anon_exclusive = false, dirty = false;
> unsigned long addr;
> int i;
>
> @@ -2098,6 +2098,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> SetPageDirty(page);
> write = pmd_write(old_pmd);
> young = pmd_young(old_pmd);
> + dirty = pmd_dirty(old_pmd);

Nitpick: This can be put under

if (pmd_dirty(old_pmd))
SetPageDirty(page);

Not a big deal.

Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

> soft_dirty = pmd_soft_dirty(old_pmd);
> uffd_wp = pmd_uffd_wp(old_pmd);
>
> @@ -2161,6 +2162,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> entry = pte_wrprotect(entry);
> if (!young)
> entry = pte_mkold(entry);
> + /* NOTE: this may set soft-dirty too on some archs */
> + if (dirty)
> + entry = pte_mkdirty(entry);
> if (soft_dirty)
> entry = pte_mksoft_dirty(entry);
> if (uffd_wp)