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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=oh3tIIHc; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0A30F606A31A; Sat, 31 Oct 2026 10:01:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=oh3tIIHc Received: by smtp.kernel.org (Postfix) with ESMTPSA id BAEAE1F000C1; Sat, 31 Oct 2026 10:01:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793440899; bh=7JYRAu4hmle3e+tmlKVwJN9rE57Jf6g2AoH9AwZHJo8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=oh3tIIHcPFh7UHKvpLtOWD5ADyL2EmR304OGIDBULi6Uu3/+uj8Ky5UM+i4Yj6Jg5 XHA3GR8+N1LxmbNZFc0pQp3U0WAknpFU26CviLyMTpmVLyHrM+izn7MmJy2jAH6D9m ioIQcwJgGz5ODBZo2aypRQJ4iX2M79ZoEh6gFnv7OJ+EuTmDklBzXi/8csanqk2vk7 ZRiJRnrZjpw/cZ9CeJX4oLX9kEYYVG4vCBnB1O7NnjfkreMeUq5smdtsopTm/wluRs vt4z59AflBiRzlthWdT0qJBpe5tblpqiCEO1IPXHUUY0+Gn2gzURUAat6VqTmw4+O2 OdvdQGUaJ0WAA== Message-ID: <75d664fb-d5c2-42c5-80a7-2316beb0ef3e@kernel.org> Date: Sat, 31 Oct 2026 11:01:30 +0100 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 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] khugepaged: fix readahead lock inversion in collapse_file() To: Lance Yang , akpm@linux-foundation.org Cc: 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, pfalcato@suse.de, ngocthang2710.1999@gmail.com, syzbot+16bf7cd0ebeb1de93aa5@syzkaller.appspotmail.com, stable@vger.kernel.org References: <20261026152013.10511-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/26/26 16:20, 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 > --- > 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); > + 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)); This create a merge conflict with my large series. I'll pick this up myself and adjust it, so Andrew please drop it. Thanks! -- Cheers, David