linux-kernel - Re: [PATCH v6 4/6] mm: change folio_lock_or_retry to use vm

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Message-ID: <CAJuCfpH7uBPS4v24MEh_4XTfJ1bz3oUhHGvtNY=XwoicXc8_XA@mail.gmail.com>
Date:   Thu, 29 Jun 2023 20:45:39 -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,
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        viro@...iv.linux.org.uk, brauner@...nel.org,
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        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v6 4/6] mm: change folio_lock_or_retry to use vm_fault directly

On Thu, Jun 29, 2023 at 8:36 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Jun 29, 2023 at 07:04:33PM -0700, Suren Baghdasaryan wrote:
> > Change folio_lock_or_retry to accept vm_fault struct and return the
> > vm_fault_t directly.
>
> I thought we decided to call this folio_lock_fault()?
>
> > +static inline vm_fault_t folio_lock_or_retry(struct folio *folio,
> > +                                          struct vm_fault *vmf)
> >  {
> >       might_sleep();
> > -     return folio_trylock(folio) || __folio_lock_or_retry(folio, mm, flags);
> > +     return folio_trylock(folio) ? 0 : __folio_lock_or_retry(folio, vmf);
>
> No, don't use the awful ternary operator.  The || form is used
> everywhere else.

Ok, but folio_trylock() returns a boolean while folio_lock_or_retry
should return vm_fault_t. How exactly do you suggest changing this?
Something like this perhaps:

static inline vm_fault_t folio_lock_or_retry(struct folio *folio,
                                          struct vm_fault *vmf)
{
     might_sleep();
     if (folio_trylock(folio))
         return 0;
     return __folio_lock_or_retry(folio, mm, flags);
}

?


>
> >  /*
> >   * Return values:
> > - * true - folio is locked; mmap_lock is still held.
> > - * false - folio is not locked.
> > + * 0 - folio is locked.
> > + * VM_FAULT_RETRY - folio is not locked.
>
> I don't think we want to be so prescriptive here.  It returns non-zero
> if the folio is not locked.  The precise value is not something that
> callers should depend on.

Ok, I'll change it to "non-zero" here.

>