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

On Thu, Jul 11, 2024 at 1:20 PM David Hildenbrand [off-list ref] wrote:

On 11.07.24 21:18, David Hildenbrand wrote:

quoted

On 11.07.24 20:56, David Hildenbrand wrote:

quoted

On 11.07.24 20:54, Jason A. Donenfeld wrote:

quoted

On Thu, Jul 11, 2024 at 08:24:07PM +0200, David Hildenbrand wrote:

quoted

quoted

And PG_large_rmappable seems to only be used for hugetlb branches.

It should be set for THP/large folios.

And it's tested too, apparently.

Okay, well, how disappointing is this below? Because I'm running out of
tricks for flag reuse.

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index b9e914e1face..c1ea49a7f198 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h

@@ -110,6 +110,7 @@ enum pageflags {
            PG_workingset,
            PG_error,
            PG_owner_priv_1,        /* Owner use. If pagecache, fs may use*/
+   PG_owner_priv_2,

Oh no, no new page flags please :)

Maybe just follow what Linux suggested: pass vma to pte_dirty() and
always return false for these special VMAs.

... or look into removing that one case that gives us headake.

No idea what would happen if we do the following:

CCing Yu Zhao.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0761f91b407f..d1dfbd4fd38d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -4280,14 +4280,9 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
                  return true;
          }

-       /* dirty lazyfree */
-       if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio)) {
-               success = lru_gen_del_folio(lruvec, folio, true);
-               VM_WARN_ON_ONCE_FOLIO(!success, folio);
-               folio_set_swapbacked(folio);
-               lruvec_add_folio_tail(lruvec, folio);
-               return true;
-       }
+       /* lazyfree: we may not be allowed to set swapbacked: MAP_DROPPABLE */
+       if (type == LRU_GEN_FILE && folio_test_anon(folio) && folio_test_dirty(folio))
+               return false;

This is an optimization to avoid an unnecessary trip to
shrink_folio_list(), so it's safe to delete the entire 'if' block, and
that would be preferable than leaving a dangling 'if'.

Note that something is unclear to me: are we maybe running into that
code also if folio_set_swapbacked() is already set and we are not in the
lazyfree path (in contrast to what is documented)?

Not sure what you mean: either rmap sees pte_dirty() and does
folio_mark_dirty() and then folio_set_swapbacked(); or MGLRU does the
same sequence, with the first two steps in walk_pte_range() and the
last one here.