2022-11-18 09:40:34

by Hugh Dickins

Subject: [PATCH 3/3] mm,thp,rmap: clean up the end of __split_huge_pmd_locked()

It's hard to add a page_add_anon_rmap() into __split_huge_pmd_locked()'s
HPAGE_PMD_NR set_pte_at() loop, without wincing at the "freeze" case's
HPAGE_PMD_NR page_remove_rmap() loop below it.

It's just a mistake to add rmaps in the "freeze" (insert migration entries
prior to splitting huge page) case: the pmd_migration case already avoids
doing that, so just follow its lead. page_add_ref() versus put_page()
likewise. But why is one more put_page() needed in the "freeze" case?
Because it's removing the pmd rmap, already removed when pmd_migration
(and freeze and pmd_migration are mutually exclusive cases).

Signed-off-by: Hugh Dickins <[email protected]>
---
mm/huge_memory.c | 15 +++++----------
1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3dee8665c585..ab5ab1a013e1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2135,7 +2135,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
uffd_wp = pmd_uffd_wp(old_pmd);

VM_BUG_ON_PAGE(!page_count(page), page);
- page_ref_add(page, HPAGE_PMD_NR - 1);

/*
* Without "freeze", we'll simply split the PMD, propagating the
@@ -2155,6 +2154,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
anon_exclusive = PageAnon(page) && PageAnonExclusive(page);
if (freeze && anon_exclusive && page_try_share_anon_rmap(page))
freeze = false;
+ if (!freeze)
+ page_ref_add(page, HPAGE_PMD_NR - 1);
}

/*
@@ -2210,27 +2211,21 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
entry = pte_mksoft_dirty(entry);
if (uffd_wp)
entry = pte_mkuffd_wp(entry);
+ page_add_anon_rmap(page + i, vma, addr, false);
}
pte = pte_offset_map(&_pmd, addr);
BUG_ON(!pte_none(*pte));
set_pte_at(mm, addr, pte, entry);
- if (!pmd_migration)
- page_add_anon_rmap(page + i, vma, addr, false);
pte_unmap(pte);
}

if (!pmd_migration)
page_remove_rmap(page, vma, true);
+ if (freeze)
+ put_page(page);

smp_wmb(); /* make pte visible before pmd */
pmd_populate(mm, pmd, pgtable);
-
- if (freeze) {
- for (i = 0; i < HPAGE_PMD_NR; i++) {
- page_remove_rmap(page + i, vma, false);
- put_page(page + i);
- }
- }
}

void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
--
2.35.3

2022-11-21 13:45:28

by Kirill A. Shutemov

Subject: Re: [PATCH 3/3] mm,thp,rmap: clean up the end of __split_huge_pmd_locked()

On Fri, Nov 18, 2022 at 01:16:20AM -0800, Hugh Dickins wrote:
> It's hard to add a page_add_anon_rmap() into __split_huge_pmd_locked()'s
> HPAGE_PMD_NR set_pte_at() loop, without wincing at the "freeze" case's
> HPAGE_PMD_NR page_remove_rmap() loop below it.
>
> It's just a mistake to add rmaps in the "freeze" (insert migration entries
> prior to splitting huge page) case: the pmd_migration case already avoids
> doing that, so just follow its lead. page_add_ref() versus put_page()
> likewise. But why is one more put_page() needed in the "freeze" case?
> Because it's removing the pmd rmap, already removed when pmd_migration
> (and freeze and pmd_migration are mutually exclusive cases).
>
> Signed-off-by: Hugh Dickins <[email protected]>

Acked-by: Kirill A. Shutemov <[email protected]>

--
Kiryl Shutsemau / Kirill A. Shutemov