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tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Sep 07, 2025 at 09:49:49PM -0700, Lokesh Gidra wrote: > Prior discussion about this can be found at [1]. > > rmap_walk() requires all folios, except non-KSM anon, to be locked. This > implies that when threads update folio->mapping to an anon_vma with > different root (currently only done by UFFDIO MOVE), they have to > serialize against rmap_walk() with write-lock on the anon_vma, hurting > scalability. Furthermore, this necessitates rechecking anon_vma when > pinning/locking an anon_vma (like in folio_lock_anon_vma_read()). > > This can be simplified quite a bit by ensuring that rmap_walk() is > always called on locked folios. Among the few callers of rmap_walk() on > unlocked anon folios, shrink_active_list()->folio_referenced() is the > only performance critical one. > > shrink_active_list() doesn't act differently depending on what > folio_referenced() returns for an anon folio. So returning 1 when it > is contended, like in case of other folio types, wouldn't have any > negative impact. > > Furthermore, as David pointed out in the previous discussion [2], this > could potentially only affect R/O pages after fork as PG_anon_exclusive > is not set. But, such folios are already isolated (prior to calling > folio_referenced()) by grabbing a reference and clearing LRU, so > do_wp_page()->wp_can_reuse_anon_folio() would not reuse such folios > anyways. > > [1] https://lore.kernel.org/all/CA*EESO4Z6wtX7ZMdDHQRe5jAAS_bQ-POq5*4aDx5jh2DvY6UHg@mail.gmail.com > [2] https://lore.kernel.org/all/dc92aef8-757f-4432-923e-70d92d13fb37@redhat.com > > CC: David Hildenbrand > CC: Lorenzo Stoakes > CC: Harry Yoo > CC: Peter Xu > CC: Suren Baghdasaryan > CC: Barry Song > CC: SeongJae Park > Signed-off-by: Lokesh Gidra > --- > mm/damon/ops-common.c | 16 ++++------------ > mm/page_idle.c | 8 ++------ > mm/rmap.c | 40 ++++++++++------------------------------ > 3 files changed, 16 insertions(+), 48 deletions(-) > > @@ -557,17 +554,6 @@ struct anon_vma *folio_lock_anon_vma_read(const struct folio *folio, > anon_vma = (struct anon_vma *) (anon_mapping - FOLIO_MAPPING_ANON); > root_anon_vma = READ_ONCE(anon_vma->root); > if (down_read_trylock(&root_anon_vma->rwsem)) { > - /* > - * folio_move_anon_rmap() might have changed the anon_vma as we > - * might not hold the folio lock here. > - */ > - if (unlikely((unsigned long)READ_ONCE(folio->mapping) != > - anon_mapping)) { > - up_read(&root_anon_vma->rwsem); > - rcu_read_unlock(); > - goto retry; > - } > - folio_lock_anon_vma_read() can be called without folio lock in a path: memory_failure() -> kill_procs_now() -> collect_procs() -> collect_procs_anon(). Not sure why collect_procs_{anon,ksm,file,fsdax} do not use rmap_walk() functionality :/ Should we take folio lock before calling kill_procs_now() in memory_failure()? -- Cheers, Harry / Hyeonggon