Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings

Jason A. Donenfeld
2024-07-11T17:09:43Z

Hi Linus, David,

On Wed, Jul 10, 2024 at 10:07:03PM -0700, Linus Torvalds wrote:
> The other approach might be to just let all the dirty handling happen
> - make droppable pages have a "page->mapping" (and not be anonymous),
> and have the mapping->a_ops->writepage() just always return success
> immediately.

When I was working on this patchset this year with the syscall, this is
similar somewhat to the initial approach I was taking with setting up a
special mapping. It turned into kind of a mess and I couldn't get it
working. There's a lot of functionality built around anonymous pages
that would need to be duplicated (I think?). I'll revisit it if need be,
but let's see if I can make avoiding the dirty bit propagation work.

> It's mainly the pte_dirty games in mm/vmscan.c that does it
> (walk_pte_range), but also the tear-down in mm/memory.c
> (zap_present_folio_ptes). Possibly others that I didn't think of.
>
> Both do have access to the vma, although in the case of
> walk_pte_range() we don't actually pass it down because we haven't
> needed it).

Actually, it's there hanging out in args->vma, and the function makes
use of that member already. So not so bad.

>
> There's also page_vma_mkclean_one(), try_to_unmap_one() and
> try_to_migrate_one(). And possibly many others I haven't even thought
> about.
>
> So quite a few places that do that "transfer dirty bit from pte to folio".

Alright, an hour later of fiddling, and it doesn't actually work (yet?)
-- the selftest fails. A diff follows below.

So, hmm... The swapbacked thing really seemed so simple... I wonder if
there's a way of recovering that.

Jason

diff --git a/mm/gup.c b/mm/gup.c
index ca0f5cedce9b..38745cc4fa06 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -990,7 +990,8 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
}
if (flags & FOLL_TOUCH) {
if ((flags & FOLL_WRITE) &&
- !pte_dirty(pte) && !PageDirty(page))
+ !pte_dirty(pte) && !PageDirty(page) &&
+ !(vma->vm_flags & VM_DROPPABLE))
set_page_dirty(page);
/*
* pte_mkyoung() would be more correct here, but atomic care
diff --git a/mm/ksm.c b/mm/ksm.c
index 34c4820e0d3d..2401fc4203ba 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1339,7 +1339,7 @@ static int write_protect_page(struct vm_area_struct *vma, struct folio *folio,
goto out_unlock;
}

- if (pte_dirty(entry))
+ if (pte_dirty(entry) && !(vma->vm_flags & VM_DROPPABLE))
folio_mark_dirty(folio);
entry = pte_mkclean(entry);

@@ -1518,7 +1518,7 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
* Page reclaim just frees a clean page with no dirty
* ptes: make sure that the ksm page would be swapped.
*/
- if (!PageDirty(page))
+ if (!PageDirty(page) && !(vma->vm_flags & VM_DROPPABLE))
SetPageDirty(page);
err = 0;
} else if (pages_identical(page, kpage))
diff --git a/mm/memory.c b/mm/memory.c
index d10e616d7389..6a02d16309be 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1479,7 +1479,7 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,

if (!folio_test_anon(folio)) {
ptent = get_and_clear_full_ptes(mm, addr, pte, nr, tlb->fullmm);
- if (pte_dirty(ptent)) {
+ if (pte_dirty(ptent) && !(vma->vm_flags & VM_DROPPABLE)) {
folio_mark_dirty(folio);
if (tlb_delay_rmap(tlb)) {
delay_rmap = true;
@@ -6140,7 +6140,8 @@ static int __access_remote_vm(struct mm_struct *mm, unsigned long addr,
if (write) {
copy_to_user_page(vma, page, addr,
maddr + offset, buf, bytes);
- set_page_dirty_lock(page);
+ if (!(vma->vm_flags & VM_DROPPABLE))
+ set_page_dirty_lock(page);
} else {
copy_from_user_page(vma, page, addr,
buf, maddr + offset, bytes);
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index aecc71972a87..72d3f8eaae6e 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -216,7 +216,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
migrate->cpages++;

/* Set the dirty flag on the folio now the pte is gone. */
- if (pte_dirty(pte))
+ if (pte_dirty(pte) && !(vma->vm_flags & VM_DROPPABLE))
folio_mark_dirty(folio);

/* Setup special migration page table entry */
diff --git a/mm/rmap.c b/mm/rmap.c
index 1f9b5a9cb121..1688d06bb617 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1397,12 +1397,7 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
VM_BUG_ON_VMA(address < vma->vm_start ||
address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
- /*
- * VM_DROPPABLE mappings don't swap; instead they're just dropped when
- * under memory pressure.
- */
- if (!(vma->vm_flags & VM_DROPPABLE))
- __folio_set_swapbacked(folio);
+ __folio_set_swapbacked(folio);
__folio_set_anon(folio, vma, address, true);

if (likely(!folio_test_large(folio))) {
@@ -1777,7 +1772,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval);

/* Set the dirty flag on the folio now the pte is gone. */
- if (pte_dirty(pteval))
+ if (pte_dirty(pteval) && !(vma->vm_flags & VM_DROPPABLE))
folio_mark_dirty(folio);

/* Update high watermark before we lower rss */
@@ -1822,7 +1817,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
}

/* MADV_FREE page check */
- if (!folio_test_swapbacked(folio)) {
+ if (!folio_test_swapbacked(folio) || (vma->vm_flags & VM_DROPPABLE)) {
int ref_count, map_count;

/*
@@ -1846,13 +1841,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
* plus the rmap(s) (dropped by discard:).
*/
if (ref_count == 1 + map_count &&
- (!folio_test_dirty(folio) ||
- /*
- * Unlike MADV_FREE mappings, VM_DROPPABLE
- * ones can be dropped even if they've
- * been dirtied.
- */
- (vma->vm_flags & VM_DROPPABLE))) {
+ !folio_test_dirty(folio)) {
dec_mm_counter(mm, MM_ANONPAGES);
goto discard;
}
@@ -1862,12 +1851,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
* discarded. Remap the page to page table.
*/
set_pte_at(mm, address, pvmw.pte, pteval);
- /*
- * Unlike MADV_FREE mappings, VM_DROPPABLE ones
- * never get swap backed on failure to drop.
- */
- if (!(vma->vm_flags & VM_DROPPABLE))
- folio_set_swapbacked(folio);
+ folio_set_swapbacked(folio);
ret = false;
page_vma_mapped_walk_done(&pvmw);
break;
@@ -2151,7 +2135,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
}

/* Set the dirty flag on the folio now the pte is gone. */
- if (pte_dirty(pteval))
+ if (pte_dirty(pteval) && !(vma->vm_flags & VM_DROPPABLE))
folio_mark_dirty(folio);

/* Update high watermark before we lower rss */
@@ -2397,7 +2381,7 @@ static bool page_make_device_exclusive_one(struct folio *folio,
pteval = ptep_clear_flush(vma, address, pvmw.pte);

/* Set the dirty flag on the folio now the pte is gone. */
- if (pte_dirty(pteval))
+ if (pte_dirty(pteval) && !(vma->vm_flags & VM_DROPPABLE))
folio_mark_dirty(folio);

/*
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2e34de9cd0d4..cf5b26bd067a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3396,6 +3396,7 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
walk->mm_stats[MM_LEAF_YOUNG]++;

if (pte_dirty(ptent) && !folio_test_dirty(folio) &&
+ !(args->vma->vm_flags & VM_DROPPABLE) &&
!(folio_test_anon(folio) && folio_test_swapbacked(folio) &&
!folio_test_swapcache(folio)))
folio_mark_dirty(folio);
@@ -3476,6 +3477,7 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long addr, struct vm_area
walk->mm_stats[MM_LEAF_YOUNG]++;

if (pmd_dirty(pmd[i]) && !folio_test_dirty(folio) &&
+ !(vma->vm_flags && VM_DROPPABLE) &&
!(folio_test_anon(folio) && folio_test_swapbacked(folio) &&
!folio_test_swapcache(folio)))
folio_mark_dirty(folio);
@@ -4076,6 +4078,7 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
young++;

if (pte_dirty(ptent) && !folio_test_dirty(folio) &&
+ !(vma->vm_flags & VM_DROPPABLE) &&
!(folio_test_anon(folio) && folio_test_swapbacked(folio) &&
!folio_test_swapcache(folio)))
folio_mark_dirty(folio);