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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> In-Reply-To: <4505a93b-2bac-4ce1-8971-4c31f1ce1362@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: C7B1F1A000D X-Stat-Signature: whkm4eo5xnhpqkxb3ij6kn5n34dfnq6a X-Rspam-User: X-HE-Tag: 1764174896-17453 X-HE-Meta: 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 znw/vlRm DN/q7LFqJoTRzdAib0CWLq+oERWV5KPzLTovaXRL8Pd51RGPMp8BnfP1C6Pax4NOM4nbc6+QR7b9P3Jocog16P72lJSkvyb2+bR4YpkM+eaZsqJSo4PDqgD8Efc+rEcUMvKcEL77TwANz0VQ7r+a6vasPBgWuXvZFArjciD/UXaBVFUjiM3F+3A0ssQ== 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 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(). Thanks, Ryan > >> >>> >>> Anyway. I am now designating you the expert at this ;) >> >> Oh no. :) >> >>> >>>> >>>>> >>>>>> >>>>>> IOW, push the READ_ONCE() down to the lowest level so the previous ones >>>>>> (that will get essentially ignore?) will get folded into the last >>>>>> READ_ONCE()? >>>>>> >>>>>> But my head still hurts and I am focusing on something else concurrently :) >>>>> >>>>> Even if we could make this work, I don't love that there's some implicit >>>>> assumption there that could easily break later on. >>>>> >>>>> I'd rather we kept it as stupid/obvious as possible... >>>> >>>> Looking at include/asm-generic/pgtable-nopmd.h I am not sure we are talking >>>> about implicit assumptions here. It's kind-of the design that the pud_t >>>> values are dummies, so why shoul the pudp_get() give you any guarantees. >>>> >>>> At least that's my current understanding, which might be very flawed :) >>> >>> I mean I'm waving my hands around like I'm working on an aircraft carrier here >>> so if you're _sure_ it's _absolutely_ safe then fine :) >> >> Well, not yet ... :) >> >