mm: zswap: free cold writeback folios promptly - #2657
Open
vfsci-bot[bot] wants to merge 3 commits into
Open
vfsci-bot[bot] wants to merge 3 commits into
vfsci-bot[bot] wants to merge 3 commits into
Conversation
This is a preparatory patch. swap_cache_alloc_folio() adds the new folio to the LRU itself, which leaves its callers no way to act on the folio before it becomes visible to reclaim. Two users need exactly that: - moving the refault evaluation out of the swap cache folio allocation requires it to happen before folio_add_lru(): that consumes PG_active to file the folio on the inactive or the active list, and under MGLRU it also reads PG_workingset to pick the generation. Setting either flag afterwards does not move the folio; - zswap writeback dropbehind needs the buffer folio to stay off the LRU entirely, as the per-CPU LRU batch would hold a reference on it and keep remove_mapping() from freeing it once writeback completes. Defer the LRU insertion to the callers and rename the helper to __swap_cache_alloc_folio(): each caller adds the folio right after the allocation, so there is no functional change intended. Suggested-by: Kairui Song <kasong@tencent.com> Reviewed-by: Kairui Song <kasong@tencent.com> Reviewed-by: Nhat Pham <nphamcs@gmail.com> Reviewed-by: Kunwu Chan <kunwu.chan@gmail.com> Acked-by: Usama Arif <usama.arif@linux.dev> Reviewed-by: Barry Song <baohua@kernel.org> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
A PG_dropbehind folio is dropped from its cache once writeback completes rather than left for reclaim to find later; this is implemented for file folios in folio_end_dropbehind(). Extend it to swap cache folios. The drop blocks on the folio lock, so it cannot run in interrupt context. Set BIO_COMPLETE_IN_TASK on the write, as the file dropbehind paths do, and drop the folio directly from folio_end_writeback(). It has to block rather than trylock: the folio is off the LRU, so skipping it would leave it in the swap cache with nothing able to reclaim it, and it cannot be put back while another thread holds its lock. Suggested-by: Yosry Ahmed <yosry@kernel.org> Suggested-by: Johannes Weiner <hannes@cmpxchg.org> Suggested-by: Nhat Pham <nphamcs@gmail.com> Reviewed-by: Nhat Pham <nphamcs@gmail.com> Reviewed-by: Kunwu Chan <kunwu.chan@gmail.com> Reviewed-by: Barry Song <baohua@kernel.org> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
zswap writeback decompresses an entry into a fresh swap cache folio and writes it back. The folio is cold by construction, yet it is left on the LRU for reclaim to find and free later, wasting a reclaim scan and keeping cold memory resident longer than necessary. Allocate the folio off the LRU and mark it PG_dropbehind so the swap dropbehind path frees it from the swap cache once writeback completes. __swap_cache_alloc_folio() evaluates a refault on the new folio, and workingset_refault() sets PG_active when it looks recent. Until now folio_add_lru() consumed that flag and __page_cache_release() cleared it once the folio left the LRU. This folio never reaches the LRU, so nothing would clear PG_active and the folio would be freed with a PAGE_FLAGS_CHECK_AT_FREE flag set, tripping bad_page() under CONFIG_DEBUG_VM. Clear it after allocation. That is a workaround: the refault should not be evaluated on a writeback buffer at all. A fix for that is on the mailing list [1]. Link: https://lore.kernel.org/linux-mm/20260911092012.92399-1-alex@ghiti.fr/ [1] Suggested-by: Johannes Weiner <hannes@cmpxchg.org> Suggested-by: Nhat Pham <nphamcs@gmail.com> Reviewed-by: Nhat Pham <nphamcs@gmail.com> Reviewed-by: Kunwu Chan <kunwu.chan@gmail.com> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
1 participant
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Series: https://patchwork.kernel.org/project/linux-fsdevel/list/?series=1170501
Submitter: Alexandre Ghiti
Version: 6
Patches: 3/3
Message-ID:
<20260921151306.625134-1-alex@ghiti.fr>Base: vfs.base.ci
Lore: https://lore.kernel.org/linux-fsdevel/20260921151306.625134-1-alex@ghiti.fr
Automated by ml2pr