From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta1.migadu.com (out-36.mta1.migadu.com [95.215.58.36]) (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 79E9D381EB9 for ; Fri, 16 Oct 2026 15:43:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.36 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792165437; cv=none; b=bHFNvqDG+PiK78faCZyiubmlO8J7zUl9NXW+U/GhAS4gBuSjB9t8Aql0Cx1B85NwqFtB8JkI0rqPggGjh96YchGTYpYQ6ruQSqXc49ILykI3OwICU3p5Kp3ViUF9S8UMJfg/gJJLMJP64oN5w/STN9YkXkjLHTmFJAqCi4RVtRY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792165437; c=relaxed/simple; bh=74rKIlxgrRlJgR/t85SrsshrUgXIRHxay9Odnbdgk5Y=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=hUSg0E1KuMU+T5Ki1bVrVxBhvobZHwPTXv+l5cWqVm5e9RGmEYD0JaPnqqU+9bawrqfD2DymLksrHOmL7rUpsD2wKzFC5TW5myqghbSV6hmAWtJ37AV5N+FBUr3+JrxVi3hkDE1Ujz6gG0Jkf7PuAYQ0FykZzQVTjLdcezLo2iY= 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=vxnDC9zU; arc=none smtp.client-ip=95.215.58.36 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="vxnDC9zU" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=74rKIlxgrRlJgR/t85SrsshrUgXIRHxay9Odnbdgk5Y=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1792165430; v=1; x=1792770230; b=vxnDC9zUUKPiBmAOIM0L+miPOrd+53bTqqQl4vMZfV6uML9mlJFKqqMkSpZj2w5cqPxhX6Iv iM56fiUM5AWOJsalqf+MUL7ktKZqBN4XJrHfHV9Pl+Mo5S6yXDFLen71WwwMl9f6NPOBnemeevt K6HAfcIvgfnqxkMLSGSqturI= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 988393ee6a1b987b; Fri, 16 Oct 2026 15:43:50 +0000 X-Mizu-Trace-ID: 988393ee6a1b987b X-Migadu-Flow: FLOW_OUT From: Lance Yang To: baolin.wang@linux.alibaba.com, pfalcato@suse.de 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: Fri, 16 Oct 2026 23:43:42 +0800 Message-ID: <20261016154342.31940-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <27e5aadc-2d68-428f-a958-308e4a75c058@linux.alibaba.com> References: <27e5aadc-2d68-428f-a958-308e4a75c058@linux.alibaba.com> 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 +Cc: Pedro 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. Cheers! >> 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()? Hmm ... we haven't taken the lock here yet, so there's just nothing to unlock, no? >> + 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 code also checks the writeback flag, so the >uptodate check seems unnecessary to me. IIRC, the folio gets unlocked even if the read fails. I just wanted to catch read errors early. And yeah, we already check uptodate later, so this one can go, thanks! >> + index = folio_next_index(folio); >> + folio_unlock(folio); >> + folio_put(folio); >> + if (result != SCAN_SUCCEED) >> + goto rollback; >> + } >> + /* drain lru cache to help folio_isolate_lru() */ >> + lru_add_drain(); >> + } > >Could we factor this code out into a small helper? Okay, let me play with it a bit :) Might pull it out if it reads cleaner. Thanks, Lance >> + >> /* >> * Ensure we have slots for all the pages in the range. This is >> * almost certainly a no-op because most of the pages must be present >> @@ -2320,17 +2348,9 @@ 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; >> - } >> + /* The range changed after the preparation pass. */ >> + result = SCAN_PAGE_LOCK; >> + goto xa_locked; >> } else if (folio_test_dirty(folio)) { >> /* >> * This page is dirty because it hasn't > >