Re: [PATCH v7 17/19] gup: Introduce FOLL_NOFAULT flag to disable page faults

[Linus Torvalds]

On Thu, Sep 9, 2021 at 4:36 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> On Fri, Aug 27, 2021 at 06:49:24PM +0200, Andreas Gruenbacher wrote:
> > Introduce a new FOLL_NOFAULT flag that causes get_user_pages to return
> > -EFAULT when it would otherwise trigger a page fault.  This is roughly
> > similar to FOLL_FAST_ONLY but available on all architectures, and less
> > fragile.
>
> So, FOLL_FAST_ONLY only has one single user through
> get_user_pages_fast_only (pin_user_pages_fast_only is entirely unused,
> which makes totally sense given that give up on fault and pin are not
> exactly useful semantics).

So I think we should treat FOLL_FAST_ONLY as a special "internal to
gup.c" flag, and perhaps not really compare it to the new
FOLL_NOFAULT.

In fact, maybe we could even just make FOLL_FAST_ONLY be the high bit,
and not expose it in <linux/mm.h> and make it entirely private as a
name in gup.c.

Because FOLL_FAST_ONLY really is meant more as a "this way we can
share code easily inside gup.c, by having the internal helpers that
*can* do everything, but not do it all when the user is one of the
limited interfaces".

Because we don't really expect people to use FOLL_FAST_ONLY externally
- they'll use the explicit interfaces we have instead (ie
"get_user_pages_fast()"). Those use-cases that want that fast-only
thing really are so special that they need to be very explicitly so.

FOLL_NOFAULT is different, in that that is something an external user
_would_ use.

Admittedly we'd only have one single case for now, but I think we may
end up with other filesystems - or other cases entirely - having that
same kind of "I am holding locks, so I can't fault into the MM, but
I'm otherwise ok with the immediate mmap_sem lock usage and sleeping".

End result: FOLL_FAST_ONLY and FOLL_NOFAULT have some similarities,
but at the same time I think they are fundamentally different.

The FAST_ONLY is the very very special "I can't sleep, I can't even
take the fundamental MM lock, and we export special interfaces because
it's _so_ special and can be used in interrupts etc".

In contrast, NOFAULT is not _that_ special. It's just another flag,
and has generic use.

               Linus