Re: [v2 PATCH] mm: gup: do not call try_grab_folio() in slow path

[Yang Shi]

On Thu, Jun 27, 2024 at 4:32 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 27 Jun 2024 19:19:40 -0400 Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> > Yang,
> >
> > On Thu, Jun 27, 2024 at 03:14:13PM -0700, Yang Shi wrote:
> > > The try_grab_folio() is supposed to be used in fast path and it elevates
> > > folio refcount by using add ref unless zero.  We are guaranteed to have
> > > at least one stable reference in slow path, so the simple atomic add
> > > could be used.  The performance difference should be trivial, but the
> > > misuse may be confusing and misleading.
> >
> > This first paragraph is IMHO misleading itself..
> >
> > I think we should mention upfront the important bit, on the user impact.
> >
> > Here IMO the user impact should be: Linux may fail longterm pin in some
> > releavnt paths when applied over CMA reserved blocks.  And if to extend a
> > bit, that include not only slow-gup but also the new memfd pinning, because
> > both of them used try_grab_folio() which used to be only for fast-gup.
>
> It's still unclear how users will be affected.  What do the *users*
> see?  If it's a slight slowdown, do we need to backport this at all?

I think Peter meant the warning reported by yangge?

Peter also mentioned the patch subject is misleading. I agree. So
how's about "mm: gup: stop abusing try_grab_folio()"?

>
> >
> > The patch itself looks mostly ok to me.
> >
> > There's still some "cleanup" part mangled together, e.g., the real meat
> > should be avoiding the folio_is_longterm_pinnable() check in relevant
> > paths.  The rest (e.g. switch slow-gup / memfd pin to use folio_ref_add()
> > not try_get_folio(), and renames) could be good cleanups.
> >
> > So a smaller fix might be doable, but again I don't have a strong opinion
> > here.
>
> The smaller the better for backporting, of course.

I view the fix to the warning as just by-product of the clean up. The
whole patch is naturally integral IMHO. We can generate a smaller fix
if it is too hard to backport. However, it should be ok since we just
need to backport to 6.6+.

>