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Received: from 30.42.72.158(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XCwX9so_1792214768 cluster:ay36) by smtp.aliyun-inc.com; Sat, 17 Oct 2026 13:26:09 +0800 Message-ID: <5b8d5b3b-9308-40d1-91f5-8c6ea045c596@linux.alibaba.com> Date: Sat, 17 Oct 2026 13:26:07 +0800 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 , pfalcato@suse.de 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: <27e5aadc-2d68-428f-a958-308e4a75c058@linux.alibaba.com> <20261016154342.31940-1-lance.yang@linux.dev> From: Baolin Wang In-Reply-To: <20261016154342.31940-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 2DF7CC0003 X-Stat-Signature: fr5mef3r7qeegg8efyxu6widph5dpwth X-Rspamd-Server: rspam05 X-HE-Tag: 1792214774-523864 X-HE-Meta: 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 L8csUPK+ 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/16/26 11:43 PM, Lance Yang wrote: > +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? Ah, yes, I misread the diff. Sorry for noise:) . >>> + 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. Because collapse_file() is already too long.