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Tue, 27 Oct 2026 16:27:25 +0000 X-Mizu-Trace-ID: 009e9fd5f7d68db1 X-Migadu-Flow: FLOW_OUT Message-ID: <137d6cc0-413d-465e-8476-a6fa14b5c2fe@linux.dev> Date: Wed, 28 Oct 2026 00:27:15 +0800 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] khugepaged: fix readahead lock inversion in collapse_file() To: "David Hildenbrand (Arm)" , Pedro Falcato Cc: akpm@linux-foundation.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 References: <20261026152013.10511-1-lance.yang@linux.dev> <321017a4-58cb-4178-8ff2-055a9bbb28b6@kernel.org> Content-Language: en-US From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: E4CF040012 X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Stat-Signature: x9fjcu3mu8swtqk118m3cf1pfhnwm5c1 X-HE-Tag: 1793118446-677892 X-HE-Meta: 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 2mL20v9E MdV4+ICPYtYM0wz9ewBJDhXx9T+SqJd2gp9E5zxT91LmHX1gRH1eVTvXu6va1XJla6m+Ft8WEqoOKZczJ3LHFaBK56w3c5fULdqbjM1+/NlMh6fYD9ioNLpvnz/EMLf3GxqtItkNYAnPoWAsuizSVHC3m1H2QDBZdzTZ76lKc2GpzagwfCpz4/2CwPL0e3Lpsd77jaeA622IqMmrtdarexg5sXPe9IbAt5MtHzATm1qniqBLtJx8OiD//OuIFuyTUPoME72DM5r1Dr57YyFU214oerJJOSg7zx9ibaeMVUqrHuUe8FEHESyatL/2rvry5Sq3stMHd7BQJSDJGzvvU1HFAiRq6DLHeYBzrLxvx6OfGM4EcFgkwUQGW+C4mmyj4G4zPtkpb+8bKZFmhF7SMM+0IwEx7PXYUaDD4ZG7F2l6S8Obeak3/M4xp9zbjIhHYK/7nlVoK9MwtI+o= 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 2026/10/27 21:58, David Hildenbrand (Arm) wrote: > On 10/27/26 14:05, Pedro Falcato wrote: >> On Tue, Oct 27, 2026 at 12:06:05PM +0100, David Hildenbrand (Arm) wrote: >>> 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. >>> >>> Makes sense. So we simply try readahead on all folios. >>> >>> Any reason we don't simply add a second loop after the first loop where we don't >>> take the folio locks and do the swapin/readahead dance, then a second loop where >>> we do all the other stuff? >> >> I'm not sure I understand. You want to nest two loops? Or do you want a loop >> upfront? I don't think we want a loop upfont. That's slow and involves >> starting/waiting for IO. Fallback logic makes more sense IMO. > > I'd have an earlier loop where we focus on filling holes, and grabbing folios > then in the second loop only from the pagecache + trylocking them. > > That would make the second easier to the eye. > > collapse_scan() file could just indicate if there are any holes and whether > looping makes sense at all. It already does a scan. > > So you could just skip the loop when there is no reason to readahead at all. > > This function is pretty horrible and the retry logic doesn't quite improve the > situation. > > But no strong opinion for this fix as long as we clean it up properly later. > > (looking forward to reworks) IIUC, we're good with the fix, so let's keep v4 as is and leave the cleanup for the rework :) Cheers, Lance