On Wed, Dec 22, 2021 at 05:08:46PM +0100, Jan Kara wrote: > On Wed 22-12-21 15:48:34, David Hildenbrand wrote: > > 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()? > > Well, but O_DIRECT reads must use FOLL_PIN in any case because they modify > page data (and so we need to detect them both for COW and filesystem needs). > O_DIRECT writes could use FOLL_GET but at this point I'm not convinced it > is worth it. Wow, I didn't realise the plan was to make FOLL_PIN the "default". I hoped it was weird crap that was going away soon. Looks like we'd better fix all the bugs in it then ...
