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Date:   Thu, 26 Nov 2020 11:24:59 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Matthew Wilcox <willy@infradead.org>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>,
        William Kucharski <william.kucharski@oracle.com>,
        Linux-FSDevel <linux-fsdevel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, Christoph Hellwig <hch@lst.de>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Yang Shi <yang.shi@linux.alibaba.com>, dchinner@redhat.com,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 00/16] Overhaul multi-page lookups for THP
In-Reply-To: <20201126121546.GN4327@casper.infradead.org>
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On Thu, 26 Nov 2020, Matthew Wilcox wrote:
> On Wed, Nov 25, 2020 at 04:11:57PM -0800, Hugh Dickins wrote:
> > > +                               index = truncate_inode_partial_page(mapping,
> > > +                                               page, lstart, lend);
> > > +                               if (index > end)
> > > +                                       end = indices[i] - 1;
> > >                         }
> > > -                       unlock_page(page);
> > 
> > The fix needed is here: instead of deleting that unlock_page(page)
> > line, it needs to be } else { unlock_page(page); }
> 
> It also needs a put_page(page);

Oh yes indeed, sorry for getting that wrong.  I'd misread the
pagevec_reinit() at the end as the old pagevec_release().  Do you
really need to do pagevec_remove_exceptionals() there if you're not
using pagevec_release()?

> 
> That's now taken care of by truncate_inode_partial_page(), so if we're
> not calling that, we need to put the page as well.  ie this:

Right, but I do find it confusing that truncate_inode_partial_page()
does the unlock_page(),put_page() whereas truncate_inode_page() does
not: I think you would do better to leave them out of _partial_page(),
even if including them there saves a couple of lines somewhere else.

But right now it's the right fix that's important: ack to yours below.

I've not yet worked out the other issues I saw: will report when I have.
Rebooted this laptop, pretty sure it missed freeing a shmem swap entry,
not yet reproduced, will study the change there later, but the non-swap 
hang in generic/476 (later seen also in generic/112) more important.

Hugh

> 
> +++ b/mm/shmem.c
> @@ -954,6 +954,9 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
>                                                 page, lstart, lend);
>                                 if (index > end)
>                                         end = indices[i] - 1;
> +                       } else {
> +                               unlock_page(page);
> +                               put_page(page);
>                         }
>                 }
>                 index = indices[i - 1] + 1;
> 
> > >                 }
> > > +               index = indices[i - 1] + 1;
> > >                 pagevec_remove_exceptionals(&pvec);
> > > -               pagevec_release(&pvec);
> > > -               index++;
> > > +               pagevec_reinit(&pvec);