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Howlett" , Julia Lawall , Nicolas Palix , Anshuman Khandual References: <6bdf2b89-7768-4b90-b5e7-ff174196ea7b@lucifer.local> <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org> <20251126134726.yrya5xxayfcde3kl@master> <6b966403-91e0-4f06-86a9-a4f7780b9557@kernel.org> <1ca9f99f-6266-47ca-8c94-1a9b9aaa717f@kernel.org> <37973e21-e8f4-4603-b93d-4e0b1b2499fa@lucifer.local> <4505a93b-2bac-4ce1-8971-4c31f1ce1362@arm.com> <150ffcb7-2df2-4f3a-a12e-9807f13c6ab9@arm.com> From: "Christophe Leroy (CS GROUP)" Content-Language: fr-FR In-Reply-To: <150ffcb7-2df2-4f3a-a12e-9807f13c6ab9@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: ib81hxawdmj5syfozb7z38y96djz48mn X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 19C5EC0014 X-HE-Tag: 1764272351-805085 X-HE-Meta: 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 J06KEVBO dNNlxiaRlHbZdySTloMRxfaqZeVhLY+tNTaTF2BFZxHuADuJj8xe7xpF2ice6ddLsmElbso8nB0eyGYZrod76JeNeePh0PhuaPQ7ZITf09MYndHmuoO/+EV07SCzJOVM1aPfXOIeCVKy/D2IBU6Lnz4RA62uhZZeFvUObvFOuBbUrzHugnREX3UakF/9Wxc/Heb7nwxBSQk+EUYUPsvyOVSNTuK2bUSJ2QPHmThT3L0ceNP8XDHBDHGYxQ3N/ckCKlEG+5pL/1O+17apDXjFPt6Nowm0013Mi6tZbFJEqZCJ489/28F2pwzX9FxP7a+Gh8zpH 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: Le 26/11/2025 à 17:34, Ryan Roberts a écrit : > On 26/11/2025 16:07, Ryan Roberts wrote: >> On 26/11/2025 15:12, David Hildenbrand (Red Hat) wrote: >>> On 11/26/25 16:08, Lorenzo Stoakes wrote: >>>> On Wed, Nov 26, 2025 at 03:56:13PM +0100, David Hildenbrand (Red Hat) wrote: >>>>> On 11/26/25 15:52, Lorenzo Stoakes wrote: >>>>>> >>>>>> Would the pmdp_get() never get invoked then? Or otherwise wouldn't that end up >>>>>> requiring a READ_ONCE() further up the stack? >>>>> >>>>> See my other reply, I think the pmdp_get() is required because all pud_* >>>>> functions are just simple stubs. >>>> >>>> OK, thought you were saying we should push further down the stack? Or up >>>> depending on how you view these things :P as in READ_ONCE at leaf? >>> >>> I think at leaf because I think the previous ones should essentially be only >>> used by stubs. >>> >>> But I haven't fully digested how this is all working. Or supposed to work. >>> >>> I'm trying to chew through the arch/arm/include/asm/pgtable-2level.h example to >>> see if I can make sense of it, >> >> I wonder if we can think about this slightly differently; >> >> READ_ONCE() has two important properties: >> >> - It guarrantees that a load will be issued, *even if output is unused* >> - It guarrantees that the read will be single-copy-atomic (no tearing) >> >> I think for the existing places where READ_ONCE() is used for pagetable reads we >> only care about: >> >> - It guarrantees that a load will be issued, *if output is used* >> - It guarrantees that the read will be single-copy-atomic (no tearing) >> >> I think if we can weaken to the "if output is used" property, then the compiler >> will optimize out all the unneccessary reads. >> >> AIUI, a C dereference provides neither of the guarrantees so that's no good. >> >> What about non-volatile asm? I'm told (thought need to verify) that for >> non-volatile asm, the compiler will emit it if the output is used and remove it >> otherwise. So if the asm contains the required single-copy-atomic, perhaps we >> are in business? >> >> So we would need a new READ_SCA() macro that could default to READ_ONCE() (which >> is stronger) and arches could opt in to providing a weaker asm version. Then the >> default pXdp_get() could be READ_SCA(). And this should work for all cases. >> >> I think. > > I'm not sure this works. It looks like the compiler is free to move non-volatile > asm sections which might be problematic for places where we are currently using > READ_ONCE() in lockless algorithms, (e.g. GUP?). We wouldn't want to end up with > a stale value. > > Another idea: > > Given the main pattern where we are aiming to optimize out the read is something > like: > > if (!pud_present(*pud)) > > where for a folded pmd: > > static inline int pud_present(pud_t pud) { return 1; } > > And we will change it to this: > > if (!pud_present(pudp_get(pud))) > > ... > > perhaps we can just define the folded pXd_present(), pXd_none(), pXd_bad(), > pXd_user() and pXd_leaf() as macros: > > #define pud_present(pud) 1 > > Then the compiler will never even see the pudp_get(). Take care about macros with unused parameters, it can be a problem sometimes, see commit 05a4ab823983 ("powerpc/uaccess: fix warning/error with access_ok()") Christophe