Re: [PATCH v3] mm/shmem: remove truncated folios from the page cache in batches

[Baolin Wang]

On 11/12/26 5:05 PM, Qiliang Yuan wrote:
> truncate_inode_pages_range() takes the folios it finds out of the page
> cache with delete_from_page_cache_batch(), which takes the inode's
> i_lock and the i_pages lock once for each batch of up to
> FOLIO_BATCH_SIZE folios.
>
> shmem_undo_range() calls truncate_inode_folio() on each folio instead,
> so filemap_remove_folio() takes and drops both locks once per folio.
> Truncating, punching or freeing a tmpfs file or memfd made of small
> folios, such as a large /dev/shm segment, pays for that on every page.
>
> Clean each folio up as truncate_inode_folio() does and, once the swap
> entries are gone, pass the batch to delete_from_page_cache_batch(),
> which needs truncate_cleanup_folio() outside mm/truncate.c. Undoing a
> failed fallocate() only removes the folios that are not uptodate, so
> keep removing those one at a time.
>
> Dropping an 8 GiB file of 4K folios in a 32 vCPU VM takes (median of
> five runs, averaged over two boots of each kernel):
>
>    operation           before (ms)  after (ms)  change
>    ftruncate() to 0           1144         901    -21%
>    hole punch                 1148         906    -21%
>    close() of a memfd         1139         912    -20%
>    unlink()                   1142         914    -20%
>
> Files of PMD-sized folios take 18-31 ms with or without this change.
>
> Signed-off-by: Qiliang Yuan <qiliang.yuan@xxxxxxxxx>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>