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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> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 5E9F8A000C X-Rspam-User: X-Rspamd-Server: rspam12 X-Stat-Signature: jton98bt5qif874p3f4ibw8rmzxx7ead X-HE-Tag: 1792165433-607639 X-HE-Meta: U2FsdGVkX19pngHgZjBMVsf2lrwTxjAHl34XBG1zuP7V3ixNlk5TLah4txrZgro3NQ4059wwx26TeRSDCQYZrERxv/Vfv4gKHk5SUZThblBrBYlZf/0TIpY9DtiuEzXqB+2MRrN18QflgwbHk83SEIE5UUojYcDEIBTpcrasYR1Sx+82pzpbBEV+iIS2zgVSaRzhbS1r4tUuYDSc1+Y/Dwutn8YTJpo4EUqH98SmlUovG7ivRKzXMee44C5SqTF2PzdMM5db5MoDmGAGrrkUWTJuFUVbVp4Akhrf1NCG9e9lsC7/pKLFfEdxilv44R7eX6oDMAT8gwjvF/CpDxVhp/qXeLEpZmTNFcZDnPd9+ZSlhNUv8Q6amTNdqM7OxSFgJHeoXePX7Vw4utNjfFkxSfW71iDTaK9/SpjxRz/3Dq5DkSDLrawxnGTDyLVVDqPjxD7I6HRefehe8IyImrC93JuZvcv62c6BZc2BpK4aEhcBHrUXP6hBpLVk90RU6CULOOT86CNi/YTLmnD/m+GlG2fhMDKtQDA5aoH+TRaG0TP4fyYGb04FvtjsZgOo89BAG6f9bY9mlFj8/4v1SBccy45bQlQsI+eiEUpk/fnC2595Et2S7eMnaU4/Rp5c/uNT5XYHEhS6lA6NbQuxuNG5G49lOP9r8wOHstZtxvtqs3hFrFB6avgxdFksrnhI9wYPATn5Fk5ZzAmmvU/qMppcyI1ImceeY7rnE+zHSKJajhRXzH4n0GU3bpeulWyRijw+SA4DT5iOzcVmPYWreP6n+Zej6+VCq15PhDOurNgR+uUvDAucsUUMDNpSi+PYL3BA/Czc/L6KmU4KUusLL5UK3ti1YYQcJwQREckt0+Sm22cgyOY5zQaJl5qT56ObWYIaiVyrvglxXM9YA811moIYtOtgL6mCdVzyMCeapMmudwyV/jKqdrY4p8oKQYyr6Qv8P5oelhWfzChlGQ0ZbbY uzGgubo+ 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: +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 > >