Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20221017161800.2003-1-vishal.moola@gmail.com> <20221017161800.2003-2-vishal.moola@gmail.com>
 <Y02JTOtYEbAyo+zu@casper.infradead.org>
In-Reply-To: <Y02JTOtYEbAyo+zu@casper.infradead.org>
From:   Vishal Moola <vishal.moola@gmail.com>
Date:   Mon, 17 Oct 2022 12:37:48 -0700
Message-ID: <CAOzc2py24=NBFX6mWZ9s0eRH-rU87n-mYsVK=TW_jtx646z_qQ@mail.gmail.com>
Subject: Re: [PATCH v3 1/2] filemap: find_lock_entries() now updates start offset
To:     Matthew Wilcox <willy@infradead.org>
Cc:     akpm@linux-foundation.org, hughd@google.com,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Mon, Oct 17, 2022 at 9:56 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Oct 17, 2022 at 09:17:59AM -0700, Vishal Moola (Oracle) wrote:
> > +++ b/mm/shmem.c
> > @@ -932,21 +932,18 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
> >
> >       folio_batch_init(&fbatch);
> >       index = start;
> > -     while (index < end && find_lock_entries(mapping, index, end - 1,
> > +     while (index < end && find_lock_entries(mapping, &index, end - 1,
>
> Sorry for not spotting this in earlier revisions, but this is wrong.
> Before, find_lock_entries() would go up to (end - 1) and then the
> index++ at the end of the loop would increment index to "end", causing
> the loop to terminate.  Now we don't increment index any more, so the
> condition is wrong.

The condition is correct. Index maintains the exact same behavior.
If a find_lock_entries() finds a folio, index is set to be directly after
the last page in that folio, or simply incrementing for a value entry.
The only time index is not changed at all is when find_lock_entries()
finds no folios, which is the same as the original behavior as well.

> I suggest just removing the 'index < end" half of the condition.

I hadn't thought about it earlier but this index < end check seems
unnecessary anyways. If index > end then find_lock_entries()
shouldn't find any folios which would cause the loop to terminate.

I could send an updated version getting rid of the "index < end"
condition as well if you would like?