From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta1.migadu.com (out-51.mta1.migadu.com [95.215.58.51]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6184B374721 for ; Sat, 17 Oct 2026 04:50:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.51 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792212647; cv=none; b=V0rjp5slgn2JiPivBaIyeg0diigjROZMQuI7pGLJLCHfawErOxZXgGc6uUYGhSDl8bvgnC1tqUhsgNtjwK8i3STiJ0ciI5R1hPeMOO+2/4w+L3ct+obPxGrsNSZYF8oDiB37F9sjMyQwCESADTPEm/b2IeWZ0oPC1Bsfo94CA8E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792212647; c=relaxed/simple; bh=DariwmLPp7xJOYKPqVbT4bqPHAX4FpqWRkGiDxZp0g4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=mgzp0vcswvdBvlRNMCwLiuSTBsPlVywI1FTDQBsbqScPBgIasQ9aQIc0DwyOvnhVXf6JPfcn6aujJHDnm8+Zqq5Eh4lyqBK9vO5+ohOLkvYqjGGmDWd4EmkyOtNgSp40PQUgpQQ9cd7a28TQFOapGNmk+svZ1x4jF9DZrtgeHj0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=ivdsxJSQ; arc=none smtp.client-ip=95.215.58.51 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="ivdsxJSQ" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=DariwmLPp7xJOYKPqVbT4bqPHAX4FpqWRkGiDxZp0g4=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1792212642; v=1; x=1792817442; b=ivdsxJSQPaqSUqZ9H/c87QzRiOuGMFexGZ0syHQ1YODe5yjB5P8tK030iiN/hfRnmxuRwOhs E2WBJxN0va5bNnFTrjFqeMo5rXMgLTAxKE1rk7YzASK2PAmqV59q79f64totFiskn25feZv0eyL UFmxpguJ3wtLlZCWE40VcLzQ= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 80e13a89da7f5256; Sat, 17 Oct 2026 04:50:42 +0000 X-Mizu-Trace-ID: 80e13a89da7f5256 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: pfalcato@suse.de, baolin.wang@linux.alibaba.com Cc: lance.yang@linux.dev, akpm@linux-foundation.org, david@kernel.org, ziy@nvidia.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, kas@kernel.org, jack@suse.cz, djwong@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, hch@lst.de, syzbot+16bf7cd0ebeb1de93aa5@syzkaller.appspotmail.com, ngocthang2710.1999@gmail.com, willy@infradead.org, rppt@kernel.org, ljs@kernel.org Subject: Re: [PATCH v3 1/1] khugepaged: fix readahead lock inversion in collapse_file() Date: Sat, 17 Oct 2026 12:50:30 +0800 Message-ID: <20261017045030.80989-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Fri, Oct 16, 2026 at 02:18:43PM +0100, Pedro Falcato wrote: >On Fri, Oct 16, 2026 at 12:00:24PM +0100, Pedro Falcato wrote: >> On Fri, Oct 16, 2026 at 05:57:48PM +0800, Baolin Wang wrote: >> > >> > >> > On 10/16/26 2:59 PM, Lance Yang wrote: >> > > collapse_file() can end up calling page_cache_sync_readahead() with >> > > source folios from earlier in the range still locked. If truncate already >> > > holds mapping->invalidate_lock exclusively and is waiting for one of >> > > those folios, readahead blocks on the same invalidate_lock and neither >> > > side can make progress. >> > > >> > > Do readahead in a separate pass before we start holding source folios >> > > locked for collapse. Keep page_cache_sync_readahead() and its existing >> > > policy, and drop each folio's lock and reference after checking that it >> > > is uptodate. If truncate or reclaim removes a folio between the two >> > > passes, roll back instead of doing readahead again with source folios >> > > locked. >> > > >> > > Shmem handles swapin and fallocated folios through shmem_get_folio() >> > > without taking mapping->invalidate_lock, so it isn't affected by this >> > > lock inversion. >> > > >> > > Reported-by: syzbot+16bf7cd0ebeb1de93aa5@syzkaller.appspotmail.com >> > > Closes: https://syzkaller.appspot.com/bug?extid=16bf7cd0ebeb1de93aa5 >> > > Fixes: 730633f0b7f9 ("mm: Protect operations adding pages to page cache with invalidate_lock") >> > > Signed-off-by: Lance Yang >> > > --- >> > >> > Thanks for the fix. >> > >> > > Thanks to Nguyen Ngoc Thang for v2. David asked for volunteers, so I'm >> > > happy to take this over. >> > > >> > > Some older stable trees lack the folio infrastructure used here, and the >> > > code will differ for each stable tree, so each backport will need quite a >> > > bit of rework and its own review. I'm not sure whether to Cc stable here. >> > > Once we agree on the fix, I'll send a separate backport for each maintained >> > > stable tree except 5.10, which isn't affected. >> > > >> > > v2 -> v3: >> > > - Replace the scan-wide invalidate_lock and page_cache_ra_unbounded() with >> > > a preparation pass using page_cache_sync_readahead(). >> > > - Roll back on a new page cache miss during collapse. >> > > - https://lore.kernel.org/all/20260913163644.122133-1-ngocthang2710.1999@gmail.com/ >> > > >> > > mm/khugepaged.c | 42 +++++++++++++++++++++++++++++++----------- >> > > 1 file changed, 31 insertions(+), 11 deletions(-) >> > > >> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> > > index ef5cd6fda34f..b3f67617caf0 100644 >> > > --- a/mm/khugepaged.c >> > > +++ b/mm/khugepaged.c >> > > @@ -2262,6 +2262,34 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, >> > > new_folio->index = start; >> > > new_folio->mapping = mapping; >> > > + if (!is_shmem) { >> > > + /* >> > > + * Populate the range before accumulating locked source folios: >> > > + * readahead may block on invalidate_lock. >> > > + */ >> > > + for (index = start; index < end;) { >> > >> > why drop xas_unlock_irq()? >> > >> > > + folio = filemap_lock_folio(mapping, index); >> > > + if (IS_ERR(folio)) { >> > > + page_cache_sync_readahead(mapping, &file->f_ra, >> > > + file, index, end - index); >> > > + folio = filemap_lock_folio(mapping, index); >> > > + if (IS_ERR(folio)) { >> > > + result = SCAN_FAIL; >> > > + goto rollback; >> > > + } >> > > + } >> > > + if (!folio_test_uptodate(folio)) >> > > + result = SCAN_FAIL; >> > >> > Do we need to check PG_uptodate after locking the folio? If we successfully >> > lock the folio, doesn't that mean the read I/O has completed? The following >> >> Yes, you do; a read can "complete" without the IO having completed successfully, >> and in those cases PG_uptodate is not set, but the folio is unlocked. >> >> Either way, why are we checking for uptodate here? We only care for starting readahead. >> Anyway, I would possibly recommend we do this as post-processing, not pre-processing. >> The current loop is a little overhead-happy. > >To clarify: I would do this kind of RA as an error path if isolation failed. >Namely, at the end of rollback, after unlocking folios. It doesn't make sense >to me to do this kind of checking upfront. I don't think it's particularly >realistic that, starting RA upfront, you'd see uptodate completed folio reads >when doing the actual isolation. Good call! I should have thought of that :P We can do this on demand after rollback rather than walk the range every time :) I played around with it a bit, something like: ---8<--- diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 85ea095906fb..ce5d89ff47b3 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -2290,6 +2290,8 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, enum scan_result result = SCAN_SUCCEED; int nr_none = 0; bool is_shmem = shmem_file(file); + bool do_readahead = false; + bool readahead_done = false; /* * MADV_COLLAPSE ignores shmem huge config, so do not check shmem @@ -2307,6 +2309,10 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, mapping_set_update(&xas, mapping); +retry: + result = SCAN_SUCCEED; + do_readahead = false; + xas_set_order(&xas, start, HPAGE_PMD_ORDER); __folio_set_locked(new_folio); if (is_shmem) __folio_set_swapbacked(new_folio); @@ -2371,17 +2377,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, } } else { /* !is_shmem */ if (!folio || xa_is_value(folio)) { - xas_unlock_irq(&xas); - page_cache_sync_readahead(mapping, &file->f_ra, - file, index, - end - index); - /* drain lru cache to help folio_isolate_lru() */ - lru_add_drain(); - folio = filemap_lock_folio(mapping, index); - if (IS_ERR(folio)) { - result = SCAN_FAIL; - goto xa_unlocked; - } + do_readahead = !readahead_done; + result = SCAN_FAIL; + goto xa_locked; } else if (folio_test_dirty(folio)) { /* * This page is dirty because it hasn't @@ -2704,6 +2702,30 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, new_folio->mapping = NULL; folio_unlock(new_folio); + + /* Readahead may take invalidate_lock; source folios are unlocked. */ + if (do_readahead) { + for (index = start; index < end;) { + folio = filemap_lock_folio(mapping, index); + if (IS_ERR(folio)) { + page_cache_sync_readahead(mapping, &file->f_ra, + file, index, end - index); + folio = filemap_lock_folio(mapping, index); + if (IS_ERR(folio)) { + result = SCAN_FAIL; + goto out_put; + } + } + index = folio_next_index(folio); + folio_unlock(folio); + folio_put(folio); + } + /* Drain the local LRU batch before retrying isolation. */ + lru_add_drain(); + readahead_done = true; + goto retry; + } +out_put: folio_put(new_folio); out: VM_BUG_ON(!list_empty(&pagelist)); -- Cheers, Lance