linux-kernel - Re: [PATCH v2 5/6] mm: implement folio wait under VMA lock

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Message-ID: <CAJuCfpE4VYz-Z4_aS3d9-8FGtQ-F4f7adYcJqRk3P3Ks7WPgQA@mail.gmail.com>
Date:   Fri, 9 Jun 2023 11:49:14 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...e.com,
        josef@...icpanda.com, jack@...e.cz, ldufour@...ux.ibm.com,
        laurent.dufour@...ibm.com, michel@...pinasse.org,
        liam.howlett@...cle.com, jglisse@...gle.com, vbabka@...e.cz,
        minchan@...gle.com, dave@...olabs.net, punit.agrawal@...edance.com,
        lstoakes@...il.com, hdanton@...a.com, apopple@...dia.com,
        peterx@...hat.com, ying.huang@...el.com, david@...hat.com,
        yuzhao@...gle.com, dhowells@...hat.com, hughd@...gle.com,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        pasha.tatashin@...een.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 5/6] mm: implement folio wait under VMA lock

On Fri, Jun 9, 2023 at 8:03 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Jun 08, 2023 at 05:51:57PM -0700, Suren Baghdasaryan wrote:
> >  static inline bool folio_lock_or_retry(struct folio *folio,
> > -             struct mm_struct *mm, unsigned int flags)
> > +             struct vm_area_struct *vma, unsigned int flags,
> > +             bool *lock_dropped)
>
> I hate these double-return-value functions.
>
> How about this for an API:
>
> vm_fault_t folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
> {
>         might_sleep();
>         if (folio_trylock(folio))
>                 return 0;
>         return __folio_lock_fault(folio, vmf);
> }
>
> Then the users look like ...
>
> > @@ -3580,8 +3581,10 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
> >       if (!folio_try_get(folio))
> >               return 0;
> >
> > -     if (!folio_lock_or_retry(folio, vma->vm_mm, vmf->flags)) {
> > +     if (!folio_lock_or_retry(folio, vma, vmf->flags, &lock_dropped)) {
> >               folio_put(folio);
> > +             if (lock_dropped && vmf->flags & FAULT_FLAG_VMA_LOCK)
> > +                     return VM_FAULT_VMA_UNLOCKED | VM_FAULT_RETRY;
> >               return VM_FAULT_RETRY;
> >       }
>
>         ret = folio_lock_fault(folio, vmf);
>         if (ret)
>                 return ret;
>
> > @@ -3837,9 +3840,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >               goto out_release;
> >       }
> >
> > -     locked = folio_lock_or_retry(folio, vma->vm_mm, vmf->flags);
> > -
> > -     if (!locked) {
> > +     if (!folio_lock_or_retry(folio, vma, vmf->flags, &lock_dropped)) {
> > +             if (lock_dropped && vmf->flags & FAULT_FLAG_VMA_LOCK)
> > +                     ret |= VM_FAULT_VMA_UNLOCKED;
> >               ret |= VM_FAULT_RETRY;
> >               goto out_release;
> >       }
>
>         ret |= folio_lock_fault(folio, vmf);
>         if (ret & VM_FAULT_RETRY)
>                 goto out_release;
>
> ie instead of trying to reconstruct what __folio_lock_fault() did from
> its outputs, we just let folio_lock_fault() tell us what it did.

Thanks for taking a look!
Ok, I think what you are suggesting is to have a new set of
folio_lock_fault()/__folio_lock_fault() functions which return
vm_fault_t directly, __folio_lock_fault() will use
__folio_lock_or_retry() internally and will adjust its return value
based on __folio_lock_or_retry()'s return and the lock releasing rules
described in the comments for __folio_lock_or_retry(). Is my
understanding correct?