>> 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. -- Thanks, David / dhildenb
