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

Jason A. Donenfeld
2024-07-11T18:08:16Z

On Thu, Jul 11, 2024 at 07:56:39PM +0200, Jason A. Donenfeld wrote:
> On Thu, Jul 11, 2024 at 07:54:34PM +0200, Jason A. Donenfeld wrote:
> > On Thu, Jul 11, 2024 at 07:27:27PM +0200, David Hildenbrand wrote:
> > > > PG_owner_priv_1 maps to PG_swapcache? :)
> > >
> > > Maybe the combination !swapbacked && swapcache could be used to indicate
> > > such folios. (we will never set swapbacked)
> > >
> > > But likely we have to be a bit careful here. We don't want
> > > folio_test_swapcache() to return for folios that ... are not in the
> > > swapcache.
> >
> > I was thinking that too, but I'm afraid it's going to be another
> > whack-a-mole nightmare. Even for things like task_mmu in procfs that
> > show stats, that's going to be wonky.
> >
> > Any other flags we can overload that aren't going to be already used in
> > our case?
>
> PG_error / folio_set_error seems unused in the non-IO case.

And PG_large_rmappable seems to only be used for hugetlb branches.

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index b9e914e1face..7fdc03197438 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -190,6 +190,7 @@ enum pageflags {
/* At least one page in this folio has the hwpoison flag set */
PG_has_hwpoisoned = PG_error,
PG_large_rmappable = PG_workingset, /* anon or file-backed */
+ PG_droppable = PG_error, /* anon droppable, not hugetlb */
};

#define PAGEFLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1)
@@ -640,6 +641,8 @@ FOLIO_TEST_CLEAR_FLAG_FALSE(young)
FOLIO_FLAG_FALSE(idle)
#endif

+FOLIO_FLAG(droppable, FOLIO_SECOND_PAGE)
+
/*
* PageReported() is used to track reported free pages within the Buddy
* allocator. We can use the non-atomic version of the test and set
diff --git a/mm/rmap.c b/mm/rmap.c
index 1f9b5a9cb121..73b4052b2f82 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1403,6 +1403,8 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
*/
if (!(vma->vm_flags & VM_DROPPABLE))
__folio_set_swapbacked(folio);
+ else
+ folio_set_droppable(folio);
__folio_set_anon(folio, vma, address, true);

if (likely(!folio_test_large(folio))) {
@@ -1852,7 +1854,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
* ones can be dropped even if they've
* been dirtied.
*/
- (vma->vm_flags & VM_DROPPABLE))) {
+ folio_test_droppable(folio))) {
dec_mm_counter(mm, MM_ANONPAGES);
goto discard;
}
@@ -1866,7 +1868,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
* Unlike MADV_FREE mappings, VM_DROPPABLE ones
* never get swap backed on failure to drop.
*/
- if (!(vma->vm_flags & VM_DROPPABLE))
+ if (!folio_test_droppable(folio))
folio_set_swapbacked(folio);
ret = false;
page_vma_mapped_walk_done(&pvmw);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2e34de9cd0d4..41340f2a12c7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4266,7 +4266,8 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
}

/* dirty lazyfree */
- if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio)) {
+ if (type == LRU_GEN_FILE && folio_test_anon(folio) &&
+ folio_test_dirty(folio) && !folio_test_droppable(folio)) {
success = lru_gen_del_folio(lruvec, folio, true);
VM_WARN_ON_ONCE_FOLIO(!success, folio);
folio_set_swapbacked(folio);