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Tue, 30 Jun 2026 19:55:13 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id 8NDfJiEfRGrtQQAAD6G6ig (envelope-from ); Tue, 30 Jun 2026 19:55:13 +0000 Date: Tue, 30 Jun 2026 20:55:12 +0100 From: Pedro Falcato To: Gregg Leventhal Cc: Alexander Viro , Christian Brauner , Jan Kara , Matthew Wilcox , Andrew Morton , Song Liu , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Eric Hagberg , David Hildenbrand , Lorenzo Stoakes , Zi Yan Subject: Re: Subject: [BUG/RFC] write-open file THP cache purge can discard dirty page cache Message-ID: References: Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="2hgan5khfldrqqbn" Content-Disposition: inline In-Reply-To: X-Spam-Flag: NO X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[multipart/mixed,text/plain,text/x-patch]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCPT_COUNT_TWELVE(0.00)[14]; MIME_TRACE(0.00)[0:+,1:+,2:+]; ARC_NA(0.00)[]; MISSING_XM_UA(0.00)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[pedro-suse.lan:mid,imap1.dmz-prg2.suse.org:helo,suse.de:email]; HAS_ATTACHMENT(0.00)[] X-Spam-Level: X-Spam-Score: -4.30 --2hgan5khfldrqqbn Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, Jun 30, 2026 at 07:49:12PM +0100, Pedro Falcato wrote: snip > > Other idea: perhaps doing filemap_write_and_wait() after the nr_thps > increment in collapse_file() will Just Work and result in a _much_ > simpler fix. And it avoids any weird forward-progress issues as no one can > write to folios at that point. > Gregg, if you could test this patch, it would be much appreciated. This patch (hopefully) makes it so no dirty folio will ever coexist with a ro-THP, thus hopefully sidestepping the entire issue in a simple way. Only compile-tested and not reviewed. -- Pedro --2hgan5khfldrqqbn Content-Type: text/x-patch; charset=us-ascii Content-Disposition: attachment; filename="0001-mm-khugepaged-write-all-dirty-folios-when-collapsing.patch" >From 43d90a937f0b24656a8d0405035c3efcfdf0961e Mon Sep 17 00:00:00 2001 From: Pedro Falcato Date: Tue, 30 Jun 2026 20:48:41 +0100 Subject: [PATCH] mm/khugepaged: write all dirty folios when collapsing Signed-off-by: Pedro Falcato --- mm/khugepaged.c | 19 ++++++++++++++++--- 1 file changed, 16 insertions(+), 3 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index a97b20617869..3f0f90ab16ba 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -60,6 +60,7 @@ enum scan_result { SCAN_STORE_FAILED, SCAN_COPY_MC, SCAN_PAGE_FILLED, + SCAN_WRITEBACK_FAIL, }; #define CREATE_TRACE_POINTS @@ -1812,7 +1813,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, pgoff_t index = 0, end = start + HPAGE_PMD_NR; LIST_HEAD(pagelist); XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER); - int nr_none = 0, result = SCAN_SUCCEED; + int nr_none = 0, result = SCAN_SUCCEED, err; bool is_shmem = shmem_file(file); VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem); @@ -2043,6 +2044,17 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, */ try_to_unmap_flush(); + /* + * If collapse looks to be successful, flush any dirty pages + * out the page cache. With the nr_thps incremented, there won't be + * any new writers (nor new dirties). + */ + if (result == SCAN_SUCCEED && !is_shmem) { + err = filemap_write_and_wait(mapping); + if (err) + result = SCAN_WRITEBACK_FAIL; + } + if (result == SCAN_SUCCEED && nr_none && !shmem_charge(mapping->host, nr_none)) result = SCAN_FAIL; @@ -2210,9 +2222,10 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, /* * Undo the updates of filemap_nr_thps_inc for non-SHMEM * file only. This undo is not needed unless failure is - * due to SCAN_COPY_MC. + * due to SCAN_COPY_MC or SCAN_WRITEBACK_FAIL. */ - if (!is_shmem && result == SCAN_COPY_MC) { + if (!is_shmem && (result == SCAN_COPY_MC || + result == SCAN_WRITEBACK_FAIL)) { filemap_nr_thps_dec(mapping); /* * Paired with the fence in do_dentry_open() -> get_write_access() -- 2.55.0 --2hgan5khfldrqqbn--