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mm: track DONTCACHE dirty pages per bdi_writeback

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Add a per-wb WB_DONTCACHE_DIRTY counter that tracks the number of dirty
pages with the dropbehind flag set (i.e., pages dirtied via RWF_DONTCACHE
writes).

Increment the counter alongside WB_RECLAIMABLE in folio_account_dirtied()
when the folio has the dropbehind flag set, and decrement it in
folio_clear_dirty_for_io() and folio_account_cleaned(). Also decrement it
when a non-DONTCACHE lookup atomically clears the dropbehind flag on a
dirty folio in __filemap_get_folio_mpol(), using folio_test_clear_dropbehind()
to prevent concurrent lookups from double-decrementing the counter, and
guarding the decrement with mapping_can_writeback() to match the increment
path.

Transfer the counter alongside WB_RECLAIMABLE in inode_do_switch_wbs() so
that the stat is properly migrated when an inode switches cgroup writeback
domains.

The counter will be used by the writeback flusher to determine how many
pages to write back when expediting writeback for IOCB_DONTCACHE writes,
without flushing the entire BDI's dirty pages.

Suggested-by: Jan Kara <jack@suse.cz>
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260511-dontcache-v7-2-2848ddce8090@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Jeff Layton 2026-05-11 07:58:28 -04:00 committed by Christian Brauner
parent 0ad186cf16
commit 88d6f128d0
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
4 changed files with 24 additions and 2 deletions
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4
fs/fs-writeback.c
View File

@ -432,6 +432,10 @@ static bool inode_do_switch_wbs(struct inode *inode,
long nr = folio_nr_pages(folio);
wb_stat_mod(old_wb, WB_RECLAIMABLE, -nr);
wb_stat_mod(new_wb, WB_RECLAIMABLE, nr);
if (folio_test_dropbehind(folio)) {
wb_stat_mod(old_wb, WB_DONTCACHE_DIRTY, -nr);
wb_stat_mod(new_wb, WB_DONTCACHE_DIRTY, nr);
}
}
}

1
include/linux/backing-dev-defs.h
View File

@ -33,6 +33,7 @@ enum wb_stat_item {
WB_WRITEBACK,
WB_DIRTIED,
WB_WRITTEN,
WB_DONTCACHE_DIRTY,
NR_WB_STAT_ITEMS
};

15
mm/filemap.c
View File

@ -2052,8 +2052,19 @@ struct folio *__filemap_get_folio_mpol(struct address_space *mapping,
if (!folio)
return ERR_PTR(-ENOENT);
/* not an uncached lookup, clear uncached if set */
if (folio_test_dropbehind(folio) && !(fgp_flags & FGP_DONTCACHE))
folio_clear_dropbehind(folio);
if (!(fgp_flags & FGP_DONTCACHE) && folio_test_clear_dropbehind(folio)) {
if (folio_test_dirty(folio) &&
mapping_can_writeback(mapping)) {
struct inode *inode = mapping->host;
struct bdi_writeback *wb;
struct wb_lock_cookie cookie = {};
long nr = folio_nr_pages(folio);
wb = unlocked_inode_to_wb_begin(inode, &cookie);
wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
unlocked_inode_to_wb_end(inode, &cookie);
}
}
return folio;
}
EXPORT_SYMBOL(__filemap_get_folio_mpol);

6
mm/page-writeback.c
View File

@ -2630,6 +2630,8 @@ static void folio_account_dirtied(struct folio *folio,
wb = inode_to_wb(inode);
lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, nr);
if (folio_test_dropbehind(folio))
wb_stat_mod(wb, WB_DONTCACHE_DIRTY, nr);
__zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, nr);
__node_stat_mod_folio(folio, NR_DIRTIED, nr);
wb_stat_mod(wb, WB_RECLAIMABLE, nr);
@ -2651,6 +2653,8 @@ void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb)
long nr = folio_nr_pages(folio);
lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
if (folio_test_dropbehind(folio))
wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
task_io_account_cancelled_write(nr * PAGE_SIZE);
@ -2920,6 +2924,8 @@ bool folio_clear_dirty_for_io(struct folio *folio)
if (folio_test_clear_dirty(folio)) {
long nr = folio_nr_pages(folio);
lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
if (folio_test_dropbehind(folio))
wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
ret = true;