From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=95.215.58.112 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793069735; b=fbh4erfG0q2QPUFAGJaMXhvs5joAgrrYrCtmJa8TaSm0H+mgb0riUuhtjXFwifvi8yCp GlzPAx4V5XD07tr6x6ZJKRdMFQe7fPz0IN7kZJ4EqR0QbsyZMCk6wnZtD74+HiEZSXIn1 gXyx1eW5E4dq91BAVN3iav62ZXv8I7SlYSk3l25NKvbj2tq11bMLtTScwlehUV2v1MyaV lrlxfOFAO/joUgnSZEi2NOODW30u2IHJ9QJaCAVe4FCQSMcjyHx0OpKR4Vnlne/+L2tka I1pSFPgbcLJXO08r/VGAMXUATcsii82Jd7CjR9pvksa034bbFWZvMC7942pqSJyz1jA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793069735; h=DMARC-Filter:X-Envelope-To:DKIM-Signature:X-Envelope-To:Received: X-Mizu-Trace-ID:X-Migadu-Flow:From:To:Cc:Subject:Date:Message-ID: X-Mailer:In-Reply-To:References:MIME-Version:Content-Type: Content-Transfer-Encoding; bh=V9PFsHDtvXgwv6NYqyvGmgu7gAiiaLp/I8Vm0DnTkkk=; b=k8OJxyuenBmeTpCQVumx/7AQmtQVnGRD/9BWTFaofIFRj0A09y5l32q00otqy1tKv8rY OUpTNEg/PpOcbrLKAqPTimF/wH0woqPL+1FGfrv4ZTmFfGXxo/GgIK+Z6ahq7S6uP+f/Z ddUKcSqwwBQa6Arv1hfwe+D3pmdCUBYhy9BjBk0NWiWV3rBNMEMpvvr536NsVpVWOgcbx TbniBhnnTIxj5mSTKVgVE8XhkxoplaDoll80Ss2SvDHE1BSHxr8dz0GbBfBp5zHRboThr SyeuEgvDypPXbsAFxo5urgvnFNjIcO9/Cj/bDh2ZnstJkxQs/7tnAIeapvAHW1HrbvQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=aBMxL8KF; arc=none smtp.remote-ip=95.215.58.112 Received: from mta1.migadu.com (out-112.mta1.migadu.com [95.215.58.112]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9F861606A31A for ; Tue, 27 Oct 2026 02:55:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 9F861606A31A 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, unprotected) header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=aBMxL8KF X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=aboeRYMBw2lMDN35kj+b7/S1t+rS41JAG+rBlUvVfhg=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1793069731; v=1; x=1793674531; b=aBMxL8KFpPbDzPXO2Idhj+x/0LL9hPc0iCPINsROyW4JAgRJKEJbI4Dne6RDhrotLAq/vasN VUh8maR/W/RnXrExj8qcQhRACKY3emUYvEyaH5rDtvXBYOdnvXLwtrGkZqXgowqsxRD5oCbyu0l 97/+ERTxKtiScXm006KKWZ8s= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 8a802e798165f00b; Tue, 27 Oct 2026 02:55:13 +0000 X-Mizu-Trace-ID: 8a802e798165f00b X-Migadu-Flow: FLOW_OUT From: Lance Yang To: pfalcato@suse.de Cc: akpm@linux-foundation.org, david@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.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, ngocthang2710.1999@gmail.com, syzbot+16bf7cd0ebeb1de93aa5@syzkaller.appspotmail.com, stable@vger.kernel.org Subject: Re: [PATCH v4] khugepaged: fix readahead lock inversion in collapse_file() Date: Tue, 27 Oct 2026 10:55:07 +0800 Message-ID: <20261027025507.59451-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: References: X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Mon, Oct 26, 2026 at 04:33:08PM +0000, Pedro Falcato wrote: >On Mon, Oct 26, 2026 at 11:20:13PM +0800, Lance Yang wrote: >> collapse_file() can call page_cache_sync_readahead() while still holding >> source folios locked. If truncate holds mapping->invalidate_lock >> exclusively and waits for one of those folios, readahead waits for >> invalidate_lock and neither side can make progress. >> >> We need to release those folio locks before starting readahead. On a page >> cache miss, roll back first, populate the collapse range, and retry once. >> Doing readahead after rollback avoids an extra pass when all folios are >> already present. If another miss occurs on retry, give up rather than keep >> retrying while truncate or reclaim removes folios. >> >> Keep page_cache_sync_readahead() and wait for read I/O with >> filemap_lock_folio(), releasing each folio before moving to the next. The >> existing uptodate check during collapse still handles read errors. >> >> Shmem uses shmem_get_folio() for swapin and fallocated folios without >> taking mapping->invalidate_lock, so it is unaffected. >> >> 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") >> Cc: stable@vger.kernel.org >> Signed-off-by: Lance Yang > >Thanks for the patch! This looks almost perfect! Thanks Pedro! > >> --- >> v3 -> v4: >> - Move readahead after rollback and retry collapse once (per Pedro). >> - Drop the preparation pass and its redundant uptodate check (per >> Baolin and Pedro). Thanks! >> - https://lore.kernel.org/all/20261016065920.4153-1-lance.yang@linux.dev/ >> >> mm/khugepaged.c | 44 +++++++++++++++++++++++++++++++++----------- >> 1 file changed, 33 insertions(+), 11 deletions(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index da91762f6358..0d3bd9e23613 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -2239,6 +2239,8 @@ 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 >> @@ -2256,6 +2258,10 @@ 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); >> @@ -2320,17 +2326,9 @@ 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 >> @@ -2657,6 +2655,30 @@ 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); > >I'm not sure if there's a point in locking the folio. If we are retrying, >then the logic above will lock it anyway and check uptodate. So I don't >think there's much of a point doing it here. Right? > >So I think we could get away with a filemap_get_folio() here, and no >filemap_lock_folio() after RA. Unless you're intent on waiting for RA >right before failing, but I don't see a point in that. Right, waiting for RA is what I intended here. Without that wait, the retry can hit folio_trylock() while the read is still in flight and bail out with SCAN_PAGE_LOCK, before it even gets to the uptodate check. If the read completes successfully, we can continue with collapse. That's why I kept the wait before retrying. Do you think waiting isn't worth it here, and we'd be better off leaving collapse to a later attempt? > >With that addressed: > >Reviewed-by: Pedro Falcato Really appreciated! >> + 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)); >> -- >> 2.49.0 >> > >-- >Pedro >