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

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Date:   Fri, 30 Jun 2023 04:47:19 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...e.com,
        josef@...icpanda.com, jack@...e.cz, ldufour@...ux.ibm.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,
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Subject: Re: [PATCH v6 4/6] mm: change folio_lock_or_retry to use vm_fault
 directly

On Thu, Jun 29, 2023 at 08:45:39PM -0700, Suren Baghdasaryan wrote:
> 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);
> }
> 
> ?

I think the automatic casting would work, but I prefer what you've
written here.

> > >  /*
> > >   * 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.

Thanks!