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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: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <150ffcb7-2df2-4f3a-a12e-9807f13c6ab9@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 38ADD140014 X-Rspamd-Server: rspam02 X-Stat-Signature: qisfro8wewpg57amufpggna4dq6akgne X-Rspam-User: X-HE-Tag: 1764189131-349632 X-HE-Meta: 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 X/OL7zad ksAW7/eFCOCQGHDkJDXbbTqLgUiWbxIosA/0wj2/HRq2ggfRMX5afpMkX/1J6ZI7Xyqjhnl3GkmKlUp0abs4isZjWFDpNnvn/BuupqasK6TZvyBCJa2mLIv8/0mkMmxKbcdTxQTwgYtIqR0mDWwQUJ/+14qBNM5zV7kYCvxIeDhne1NnRtRU7TJD6jqoQxD585GXFcyVgJ6XYnSzrpef7GAVO0y0Ea8EXC+Nuyoo44YN0C1dE87lFZsYUeF3EdDO9OpOZN+6k0B4gzQAHuOpv05IUhehKuuLQfLU3aAK/Ybqbyzc= 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 11/26/25 17:34, Ryan Roberts wrote: > 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 > Let's take a step back and realize that with __PAGETABLE_PMD_FOLDED (a) *pudp does not make any sense For a folded PMD, *pudp == *pmdp and consequently we would actually get a PMD, not a PUD. For this reason all these pud_* helpers ignore the passed value completely. It would be wrong. (b) pmd_offset() does *not* consume a pud but instead a pudp. That makes sense, just imagine what would happen if someone would pass *pudp to that helper (we'd dereference twice ...). So I wonder if we can just teach get_pudp() and friends to ... return true garbage instead of dereferencing something that does not make sense? diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h index 32e8457ad5352..c95d0d89ab3f1 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -351,7 +351,13 @@ static inline pmd_t pmdp_get(pmd_t *pmdp) #ifndef pudp_get static inline pud_t pudp_get(pud_t *pudp) { +#ifdef __PAGETABLE_PMD_FOLDED + pud_t dummy = { 0 }; + + return dummy; +#else return READ_ONCE(*pudp); +#endif } #endif set_pud/pud_page/pud_pgtable helper are confusing, I would assume they are essentially unused (like documented for set_put) and only required to keep compilers happy. -- Cheers David