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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-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: xn9q1dduwkqmzcchd183hfxq1pyixhop X-Spam-Status: No, score=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: E594F20005 X-Rspam-User: X-HE-Tag: 1793069732-157394 X-HE-Meta: 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 rxSMwOAH 2WdynJMT+wvMeyZ9FV5GdN5K8N04vX1/2RF90GbCMygdQCyUDgtPtmao4I0oIIQgedN5paX4SO77TI5B1YpXpP/St7Im+LYfS+G9+1umvJco1hs4tb28wJvjlkpVQUWrJSLuqAfOOZ9KCxOMyXYZUviPPxvPxxOQHKdvVPcckM4kZTTQrktA1sqKB2UCGn/lW0vLZfJUzdAVRkqydUTRnAIxExpC3xBAtgmv5HrNLukV1hbX640fV6EUQ48v+7C/Uhuvy48+ED5TLI7/c/UdeA+cUcAiDkUPkzv7IeSLBJYNXS0XEqMTAPfgBOE0CLz3dVQxzJWbUQVjkXxgYKIMmarYt/eqX+UTt14wt4KF+vxxJtGo9yOFNsvdkoBKFegWyxoz87seNa59fRsER8RmPmVBUbFX0Z7atbMyzPJJvGEMQbim86fJ9onJEsogsfSyf9NxCcjpvWgDpeWhqaKQMmDfNPSb8FR9F9//TaoAeIBnxIa+fWicE+YcClEWr31+8Z19iR1hXJOSyOcYKyG5hHt4iO95t2Ru5dlo6ueD3nwvjumLqdiofblMEsQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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 >