Re: [PATCH v2 4/4] mm: Avoid splitting pmd for lazyfree pmd-mapped THP in try_to_unmap
From: Barry Song
Date: Tue Jan 14 2025 - 01:01:18 EST
> > > if (!pvmw.pte) {
> > > + lazyfree = folio_test_anon(folio) && !folio_test_swapbacked(folio);
> >
> > You've checked lazyfree here, so can we remove the duplicate check in
> > unmap_huge_pmd_locked()? Then the code should be:
> >
> > if (lazyfree && unmap_huge_pmd_locked(...))
> > goto walk_done;
>
>
> right. it seems unmap_huge_pmd_locked() only handles lazyfree pmd-mapped
> thp. so i guess the code could be:
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index aea49f7125f1..c4c3a7896de4 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3131,11 +3131,10 @@ bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
> VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
> + VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
> + VM_WARN_ON_FOLIO(folio_test_swapbacked(folio), folio);
>
> - if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
> - return __discard_anon_folio_pmd_locked(vma, addr, pmdp, folio);
> -
> - return false;
> + return __discard_anon_folio_pmd_locked(vma, addr, pmdp, folio);
> }
>
> static void remap_page(struct folio *folio, unsigned long nr, int flags)
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 02c4e4b2cd7b..72907eb1b8fe 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1671,7 +1671,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
> pte_t pteval;
> struct page *subpage;
> - bool anon_exclusive, lazyfree, ret = true;
> + bool anon_exclusive, ret = true;
> struct mmu_notifier_range range;
> enum ttu_flags flags = (enum ttu_flags)(long)arg;
> int nr_pages = 1;
> @@ -1724,18 +1724,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> }
>
> if (!pvmw.pte) {
> - lazyfree = folio_test_anon(folio) && !folio_test_swapbacked(folio);
> -
> - if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
> - folio))
> - goto walk_done;
> - /*
> - * unmap_huge_pmd_locked has either already marked
> - * the folio as swap-backed or decided to retain it
> - * due to GUP or speculative references.
> - */
> - if (lazyfree)
> + if (folio_test_anon(folio) && !folio_test_swapbacked(folio)) {
> + if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd, folio))
> + goto walk_done;
> + /*
> + * unmap_huge_pmd_locked has either already marked
> + * the folio as swap-backed or decided to retain it
> + * due to GUP or speculative references.
> + */
> goto walk_abort;
> + }
>
> if (flags & TTU_SPLIT_HUGE_PMD) {
> /*
>
> >
> > > if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
> > > folio))
> > > goto walk_done;
> > > + /*
> > > + * unmap_huge_pmd_locked has either already marked
> > > + * the folio as swap-backed or decided to retain it
> > > + * due to GUP or speculative references.
> > > + */
> > > + if (lazyfree)
> > > + goto walk_abort;
> > >
> > > if (flags & TTU_SPLIT_HUGE_PMD) {
> > > /*
The final diff is as follows.
Baolin, do you have any additional comments before I send out v3?
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3d3ebdc002d5..47cc8c3f8f80 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3070,8 +3070,12 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma,
int ref_count, map_count;
pmd_t orig_pmd = *pmdp;
- if (folio_test_dirty(folio) || pmd_dirty(orig_pmd))
+ if (pmd_dirty(orig_pmd))
+ folio_set_dirty(folio);
+ if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
+ folio_set_swapbacked(folio);
return false;
+ }
orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp);
@@ -3098,8 +3102,15 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma,
*
* The only folio refs must be one from isolation plus the rmap(s).
*/
- if (folio_test_dirty(folio) || pmd_dirty(orig_pmd) ||
- ref_count != map_count + 1) {
+ if (pmd_dirty(orig_pmd))
+ folio_set_dirty(folio);
+ if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
+ folio_set_swapbacked(folio);
+ set_pmd_at(mm, addr, pmdp, orig_pmd);
+ return false;
+ }
+
+ if (ref_count != map_count + 1) {
set_pmd_at(mm, addr, pmdp, orig_pmd);
return false;
}
@@ -3119,12 +3130,11 @@ bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
{
VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
+ VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
+ VM_WARN_ON_FOLIO(folio_test_swapbacked(folio), folio);
VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
- if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
- return __discard_anon_folio_pmd_locked(vma, addr, pmdp, folio);
-
- return false;
+ return __discard_anon_folio_pmd_locked(vma, addr, pmdp, folio);
}
static void remap_page(struct folio *folio, unsigned long nr, int flags)
diff --git a/mm/rmap.c b/mm/rmap.c
index 3ef659310797..72907eb1b8fe 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1724,9 +1724,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
}
if (!pvmw.pte) {
- if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
- folio))
- goto walk_done;
+ if (folio_test_anon(folio) && !folio_test_swapbacked(folio)) {
+ if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd, folio))
+ goto walk_done;
+ /*
+ * unmap_huge_pmd_locked has either already marked
+ * the folio as swap-backed or decided to retain it
+ * due to GUP or speculative references.
+ */
+ goto walk_abort;
+ }
if (flags & TTU_SPLIT_HUGE_PMD) {
/*
--
2.39.3 (Apple Git-146)