From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-99.freemail.mail.aliyun.com (out30-99.freemail.mail.aliyun.com [115.124.30.99]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AF5C23612ED for ; Sun, 18 Oct 2026 02:47:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.99 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792291660; cv=none; b=u4BFc+skE6q0ctL70v5uRnBu9FwynJK4SOGGKOv+TdT5erJy02YDRdjJXnCeriW1ok6D6bCooaBOBxe0wvmOu4jqCwWj8SKdj5u0yZMEtukbihMibf3oZv6qfNcA0q2yjDa3ysTlC/Wc3QpAcuz0rpdJgJ1jl1I/cldWRWj0fNU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792291660; c=relaxed/simple; bh=FRvSS77tPkxhqI8ApcJrTJap+T/z8Z3hcT8InsFgR2U=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mM7QNm8udORj3kZyX/8u/VykEtkWdOkcZhduUSeCcGHECbCL81o+DDcrQZCuvhhKrBqA7s5WO8BJwh5xJHx1gozcjFWAnheSqZrpvz4OS2EjurboHUBfD7Kk0USvw7mieEghkOS5V/n3uIdyHDDFQ8IWrOF0BBT+EeEcLO/wbc0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=bYly3sO1; arc=none smtp.client-ip=115.124.30.99 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="bYly3sO1" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1792291654; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=Z6vhl9H/bI+MfnZt/1kQc+tu5ODLFIoF8c4dyDq9S+M=; b=bYly3sO1JcHwB0ZNr0WJ7REYzk7DaDX8vspWu0TIFgSlAovtdsjJVfcK/CC6S27TDfVQjMaD095VEFAUXRM8WiwSbrioc3fgignu8WFpdrGANqillbJmEz4VmMeq3nZayDdRYYszorkCd1Bbu5hHG4LWcWh9+WGKoHy9QDPNhY0= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R471e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033037033178;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=22;SR=0;TI=SMTPD_---0XCyL-xi_1792291651; Received: from 30.42.72.158(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XCyL-xi_1792291651 cluster:ay36) by smtp.aliyun-inc.com; Sun, 18 Oct 2026 10:47:32 +0800 Message-ID: Date: Sun, 18 Oct 2026 10:47:31 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/1] khugepaged: fix readahead lock inversion in collapse_file() To: Lance Yang , Pedro Falcato Cc: 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 References: <20261016065920.4153-1-lance.yang@linux.dev> <27e5aadc-2d68-428f-a958-308e4a75c058@linux.alibaba.com> <6a005d89-df5b-4524-a4b9-c637cb15f1ee@linux.alibaba.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10/17/26 3:41 PM, Lance Yang wrote: > > > On 2026/10/17 13:43, Baolin Wang wrote: >> >> >> On 10/16/26 9:18 PM, 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. >> >> OK. >> >>>> 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. >> >> Sounds good. >> >>> 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. >> >> But why not add RA logic in collapse_pmd(), as we do for >> SCAN_PAGE_DIRTY_OR_WRITEBACK? There's already retry logic there, and >> this would also simplify the long collapse_file(). > > That would be cleaner, yeah. Some older stable trees don't have that > outer retry loop, though ... I'd prefer to get the fix backported first, > then move the RA/retry logic out in a follow-up, wdyt? IIRC, the approach to bug fixes is to fix bugs properly and cleanly for upstream, without being constrained by backporting requirements. Anyway, I don't have a strong opinion on this. Let's also see what David prefers.