linux-kernel - Re: [PATCH v3 1/2] filemap: find_lock_entries() now updates start offset

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Date:   Mon, 17 Oct 2022 12:37:48 -0700
From:   Vishal Moola <vishal.moola@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, hughd@...gle.com,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] filemap: find_lock_entries() now updates start offset

On Mon, Oct 17, 2022 at 9:56 AM Matthew Wilcox <willy@...radead.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?