On 22.12.21 15:42, Jan Kara wrote: > On Wed 22-12-21 14:09:41, David Hildenbrand wrote: >>>> IIUC, our COW logic makes sure that a shared anonymous page that might >>>> still be used by a R/O FOLL_GET cannot be modified, because any attempt >>>> to modify it would result in a copy. >>> >>> Well, we defined FOLL_PIN to mean the intent that the caller wants to access >>> not only page state (for which is enough FOLL_GET and there are some users >>> - mostly inside mm - who need this) but also page data. Eventually, we even >>> wanted to make FOLL_GET unavailable to broad areas of kernel (and keep it >>> internal to only MM for its dirty deeds ;)) to reduce the misuse of GUP. >>> >>> For file pages we need this data vs no-data access distinction so that >>> filesystems can detect when someone can be accessing page data although the >>> page is unmapped. Practically, filesystems care most about when someone >>> can be *modifying* page data (we need to make sure data is stable e.g. when >>> writing back data to disk or doing data checksumming or other operations) >>> so using FOLL_GET when wanting to only read page data should be OK for >>> filesystems but honestly I would be reluctant to break the rule of "use >>> FOLL_PIN when wanting to access page data" to keep things simple and >>> reasonably easy to understand for parties such as filesystem developers or >>> driver developers who all need to interact with pinned pages... >> >> Right, from an API perspective we really want people to use FOLL_PIN. >> >> To optimize this case in particular it would help if we would have the >> FOLL flags on the unpin path. Then we could just decide internally >> "well, short-term R/O FOLL_PIN can be really lightweight, we can treat >> this like a FOLL_GET instead". And we would need that as well if we were >> to keep different counters for R/O vs. R/W pinned. > > Well, I guess the question here is: Which GUP user needs only R/O access to > page data and is so performance critical that it would be worth it to > sacrifice API clarity for speed? I'm not aware of any but I was not looking > really hard... I'd be interested in examples as well. Maybe databases that use O_DIRECT after fork()? -- Thanks, David / dhildenb
