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

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Date:   Mon, 17 Oct 2022 20:43:46 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Vishal Moola <vishal.moola@...il.com>
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 12:37:48PM -0700, Vishal Moola wrote:
> 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.

Uh, right.  I had the wrong idea in my head that index wouldn't increase
past end-1, but of course it can.

> > 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?

Something to consider is that if end is 0 then end-1 is -1, which is
effectively infinity, and we'll do the wrong thing?  So maybe just
leave it alone, and go with v3 as-is?