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Wed, 15 Oct 2025 16:08:48 +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 DR0YHxDH72hOagAAD6G6ig (envelope-from ); Wed, 15 Oct 2025 16:08:48 +0000 Message-ID: <421c7c42-bf7d-4277-b364-525c63254205@suse.cz> Date: Wed, 15 Oct 2025 18:08:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/page_alloc: simplify and cleanup pcp locking To: Joshua Hahn Cc: Andrew Morton , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20251015145143.3001503-1-joshua.hahnjy@gmail.com> Content-Language: en-US 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: <20251015145143.3001503-1-joshua.hahnjy@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: qeb19bq8ykcu838u6kqhago9uujkios4 X-Rspamd-Queue-Id: 58C51A0007 X-Rspamd-Server: rspam09 X-HE-Tag: 1760544530-731219 X-HE-Meta: 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 HGqx3wiz eIeokkV5Ja5SMznxFUOLrg/nzXO3bHNT3AMkl9v4n7ZrF6aM4Acj/WFmAVLOsTN4Ln/K4kw09M56w1HpEFl6LUE0CdQ== 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 10/15/25 16:51, Joshua Hahn wrote: > On Wed, 15 Oct 2025 11:36:09 +0200 Vlastimil Babka wrote: > >> The pcp locking relies on pcp_spin_trylock() which has to be used >> together with pcp_trylock_prepare()/pcp_trylock_finish() to work >> properly on !SMP !RT configs. This is tedious and error-prone. >> >> We can remove pcp_spin_lock() and underlying pcpu_spin_lock() because we >> don't use it. Afterwards pcpu_spin_unlock() is only used together with >> pcp_spin_trylock(). Therefore we can add the UP_flags parameter to them >> and handle pcp_trylock_prepare()/finish() within them. >> >> Additionally for the configs where pcp_trylock_prepare() is a no-op (SMP >> || RT) make it pass &UP_flags to a no-op inline function. This ensures >> typechecking and makes the local variable "used" so we can remove the >> __maybe_unused attributes. >> >> In my compile testing, bloat-o-meter reported no change on SMP config, >> so the compiler is capable of optimizing away the no-ops same as before, >> and we have simplified the code using pcp_spin_trylock(). >> >> Signed-off-by: Vlastimil Babka > > Hello Vlastimil, I hope you are doing well! > > Thank you for this patch. This is a pattern that I found quite cumbersome, > so this patch really makes the code so much easier to understand and read. Hi, that's good to hear! >> --- >> based on mm-new >> --- >> mm/page_alloc.c | 99 +++++++++++++++++++++++---------------------------------- >> 1 file changed, 40 insertions(+), 59 deletions(-) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 0155a66d7367..2bf707f92d83 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -99,9 +99,12 @@ static DEFINE_MUTEX(pcp_batch_high_lock); >> /* >> * On SMP, spin_trylock is sufficient protection. >> * On PREEMPT_RT, spin_trylock is equivalent on both SMP and UP. >> + * Pass flags to a no-op inline function to typecheck and silence the unused >> + * variable warning. >> */ >> -#define pcp_trylock_prepare(flags) do { } while (0) >> -#define pcp_trylock_finish(flag) do { } while (0) >> +static inline void __pcp_trylock_prepare(unsigned long *flags) { } >> +#define pcp_trylock_prepare(flags) __pcp_trylock_prepare(&(flags)) >> +#define pcp_trylock_finish(flags) do { } while (0) >> #else > > I have one question here. I was a bit unsure why we do the typechecking and > silencing for the unused variable warning for only pcp_trylock_prepare, but > not for pcp_trylock_finish. Is it because pcp_trylock_finish will always > be called after pcp_trylock_prepare, so the flag will have been used at > that point? Exactly. > I was concerned that there would have been some area where only > pcp_trylock_finish would have been used, but compiling with W=1 seems to show > no errors on my end : -) So it looks good to me! Feel free to add: Yeah we can change that if ever we end up with some code that needs it (hopefully not). > Reviewed-by: Joshua Hahn Thanks! > Thank you! I hope you have a great day! > Joshua