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 04:36:44 +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,
        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 v6 4/6] mm: change folio_lock_or_retry to use vm_fault
 directly

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.

>  /*
>   * 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.