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b=nzbWYI9vKI/tTDUA+H9ofYVeqMcDs9WFXnQxapTZvclGE1hgFml0C4R4jzSpqtKJr 4MdQr23HDPHbhPWfOHlpbdkMSfvFW7U7dp2+f+CTrwfw+d+oDEwbQizRruYkFAAI9N L++c8R1O3e4tn4B4d3jkdkEIaq9SlIY9iWicWz52cw1yOyoCe0rzoOifjvAs9tqDLg i7+DCKlkiAafmVZ6c5zcMCHLYwdyxgovbSaDz1YshG92cMXd42jSmBeqSqNpd+WYWV H6h3HVHc637r1NlyaS3HfK5DcRfID+ZcMUio7YjcEJ3XvpvyVzb1GdLt45UNs15Gum 7NUjIRr94h+lg== From: Mike Snitzer To: Chuck Lever , Jeff Layton Cc: linux-nfs@vger.kernel.org, NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Christoph Hellwig , Jens Axboe , Matthew Wilcox , linux-mm@kvack.org Subject: [PATCH v4 07/10] mm: add folio_mark_dropbehind() to mark a folio dropbehind and account it Date: Thu, 15 Oct 2026 19:33:06 -0400 Message-ID: <20261015233309.42999-8-snitzer@kernel.org> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20261015233309.42999-1-snitzer@kernel.org> References: <20261015233309.42999-1-snitzer@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 3F97E180005 X-Stat-Signature: 9uh3oirfj5bih56mgdyfsrdze6y9jfkc X-HE-Tag: 1792107200-462795 X-HE-Meta: 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 UNTGy2Lr oEWnAFk6PxFmWMvexsajj2dtRe15+2Lzgm+aBYvF6aN4oWTbOoyf7mYKAsEgksOxqK3CxnOGi7S0ZevzDRTBRDtR5qKkzFPGxgXBxdkwlg15MoLb1HI7bmB2PDpL32UgNpBZ5Ez5UBzjfFipkRd/yjKSr8NphUDG5Q7PiCErnZKfL18JTFsIhEFieKFXxGQtQnZYJQcbL9YK1h6VoC6NWDgBoJ85UPJGyvFJBjWe1zPRbJ8R4CUxe2JFt++EMM7XSGsYRWwCH6e/K2lvMndqfxmJoI4HJ2fmCWX1YSfl5yXrbDMIMwCYP/tr7kKlS5EiPv5s6ggcK5KLCPmu21HRx0Gyz9yWJR5qg8C7MPtUOtQQMOnjryTFbYm0xj5aSJjGGak5c2cQBnI7g7HVp6rKESsNUPktxphU924AhGD8pZDLMq7kIcxPf+nnKbkrwA12FUFigYfztr+SF85vHcR9Jr3tdALzeg25q/93U Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: A folio allocated by an RWF_DONTCACHE write is marked dropbehind before it is dirtied, so folio_account_dirtied() adds it to WB_DONTCACHE_DIRTY and folio_clear_dirty_for_io() or folio_account_cleaned() subtracts it when it is cleaned. Nothing marks a folio that already exists. A caller that sets PG_dropbehind on a folio that is already dirty unbalances the counter: the clean subtracts what was never added, and once the counter goes negative, wb_stat_sum() reads it as zero and the DONTCACHE writeback kick stops writing anything back for every writer on that bdi_writeback. Add folio_mark_dropbehind(), the inverse of the clear in __filemap_get_folio_mpol(): with the folio locked, mark it and, if it is dirty, add it to WB_DONTCACHE_DIRTY. The folio lock orders this against dirtying and against folio_clear_dirty_for_io(). A concurrent lookup without FGP_DONTCACHE that clears the flag again subtracts what this added. The name follows folio_set_dirty() and folio_mark_dirty(): the first only sets PG_dirty, the second also accounts the dirty folio. folio_set_dropbehind() likewise only sets PG_dropbehind, and folio_mark_dropbehind() also accounts it in WB_DONTCACHE_DIRTY. A later commit uses it in NFSD to drop a page shared by two misaligned WRITEs once both have written it. Cc: Jeff Layton Cc: Jens Axboe Cc: Matthew Wilcox (Oracle) Cc: linux-mm@kvack.org Assisted-by: LLM Signed-off-by: Mike Snitzer --- include/linux/pagemap.h | 1 + mm/filemap.c | 31 +++++++++++++++++++++++++++++++ 2 files changed, 32 insertions(+) diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 0adfa6605653d..3012bd95812a1 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -1310,6 +1310,7 @@ void end_page_writeback(struct page *page); void folio_end_writeback(struct folio *folio); void folio_end_writeback_no_dropbehind(struct folio *folio); void folio_end_dropbehind(struct folio *folio); +void folio_mark_dropbehind(struct folio *folio); void folio_wait_stable(struct folio *folio); void __folio_mark_dirty(struct folio *folio, struct address_space *, int warn); void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb); diff --git a/mm/filemap.c b/mm/filemap.c index 00fd89cf6f550..ed7e4dbea653f 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1644,6 +1644,37 @@ void folio_end_dropbehind(struct folio *folio) } EXPORT_SYMBOL_GPL(folio_end_dropbehind); +/** + * folio_mark_dropbehind - mark an existing folio dropbehind + * @folio: the folio, locked by the caller + * + * Mark @folio so that the writeback that cleans it drops it. A folio that + * is already dirty is added to WB_DONTCACHE_DIRTY, as folio_account_dirtied() + * would have added it had it been marked when dirtied, so that the clean + * that subtracts it leaves the counter balanced. + */ +void folio_mark_dropbehind(struct folio *folio) +{ + struct address_space *mapping = folio->mapping; + + VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio); + + if (folio_test_dropbehind(folio)) + return; + folio_set_dropbehind(folio); + if (mapping && folio_test_dirty(folio) && + mapping_can_writeback(mapping)) { + struct inode *inode = mapping->host; + struct bdi_writeback *wb; + struct wb_lock_cookie cookie = {}; + + wb = unlocked_inode_to_wb_begin(inode, &cookie); + wb_stat_mod(wb, WB_DONTCACHE_DIRTY, folio_nr_pages(folio)); + unlocked_inode_to_wb_end(inode, &cookie); + } +} +EXPORT_SYMBOL_GPL(folio_mark_dropbehind); + /** * folio_end_writeback_no_dropbehind - End writeback against a folio. * @folio: The folio. -- 2.52.0