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Wed, 12 Nov 2025 10:53:40 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id crmgCTRnFGn5VAAAD6G6ig (envelope-from ); Wed, 12 Nov 2025 10:53:40 +0000 Message-ID: Date: Wed, 12 Nov 2025 11:53:39 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 6/7] mempool: fix a wakeup race when sleeping for elements Content-Language: en-US To: Christoph Hellwig , Andrew Morton Cc: Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Eric Biggers , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20251111135300.752962-1-hch@lst.de> <20251111135300.752962-7-hch@lst.de> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: <20251111135300.752962-7-hch@lst.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 1A408160008 X-Stat-Signature: 9jbkp46kem81jd65qzqfbethr4fn6qtb X-Rspamd-Server: rspam02 X-Rspam-User: X-HE-Tag: 1762944821-521502 X-HE-Meta: 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 Vc/j+s5e p7af6kgKa6aDoqLGa3tFfr/B7V9c/ohlA8x4fFmpl6y29kZ+edT5fBUA0JzYJzftCoHsD9psa5SdlMNQ410Ow4G0Enbi2cNCEOBkDETmMNH1JCW2D2YMzUUWu7fbNo9WPqRrlLO85QgtlEBdjPRkMTec5SupQKfFS2L2m04oWJCx2ZyJC9LQQlxPLNetfjRib4angH1oCmQgZroSv7j7Yi7Oy4GM0ehXq7CvLbSmDrMbtbLirs+dZxoyK72wgxqkBmTCc6pWioqpio33guV0+xEA+Iu4TjPmnpr0P X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 11/11/25 14:52, Christoph Hellwig wrote: > Waiters always need to re-check their condition after adding themselves > to the waitqueue, as otherwise they might miss a wakeup. Check > for elements in the pool and use them before going to sleep. > > The workaround mentioned was probably due to this, but seems genuinely > useful for other reasons, so keep it and update the comment describing > it. Thanks for looking into that historic comment :) > Signed-off-by: Christoph Hellwig > --- > mm/mempool.c | 13 +++++++++---- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/mm/mempool.c b/mm/mempool.c > index 850362f4ca7a..8cf3b5705b7f 100644 > --- a/mm/mempool.c > +++ b/mm/mempool.c > @@ -388,6 +388,7 @@ static void *mempool_alloc_from_pool(struct mempool *pool, gfp_t gfp_mask) > spin_lock_irqsave(&pool->lock, flags); > if (unlikely(!pool->curr_nr)) > goto fail; > +alloc: > element = remove_element(pool); > spin_unlock_irqrestore(&pool->lock, flags); > > @@ -406,13 +407,17 @@ static void *mempool_alloc_from_pool(struct mempool *pool, gfp_t gfp_mask) > DEFINE_WAIT(wait); > > prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE); > + if (pool->curr_nr) { > + finish_wait(&pool->wait, &wait); > + goto alloc; > + } > spin_unlock_irqrestore(&pool->lock, flags); I think the race already cannot exist, thanks to the pool->lock being unlocked after prepare_to_wait()? The freeing path can't bump pool->curr_nr without the lock, so the condition you added can't even be true, no? > > /* > - * Wait for someone else to return an element to @pool. > - * > - * FIXME: this should be io_schedule(). The timeout is there as > - * a workaround for some DM problems in 2.6.18. > + * Wait for someone else to return an element to @pool, but wake > + * up occasionally as memory pressure might have reduced even > + * and the normal allocation in alloc_fn could succeed even if > + * no element was returned. > */ > io_schedule_timeout(5 * HZ); > finish_wait(&pool->wait, &wait);