Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

[Jan Kara]

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