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... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
