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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 15 Jan 2019 16:39:39 +0100
Message-ID: <CAJfpegvKxJ06hfXR5yxWkdz0GzTw7vR2x8E5bjdO1df1J1C54w@mail.gmail.com>
Subject: Re: [PATCH 2/2] fuse: Replace page without copying in fuse_writepage_in_flight()
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
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On Mon, Nov 26, 2018 at 10:46 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>
> It looks like we can optimize old_req page replacement
> and avoid copying by simple updating the request's page.
>
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> ---
>  fs/fuse/file.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index c6650c68b31a..83b54b082c86 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1778,7 +1778,7 @@ static bool fuse_writepage_in_flight(struct fuse_req *new_req,
>         if (old_req->num_pages == 1 && old_req != first_req) {
>                 struct backing_dev_info *bdi = inode_to_bdi(page->mapping->host);
>
> -               copy_highpage(old_req->pages[0], page);
> +               swap(old_req->pages[0], page);

This would mess up refcounting for all pages involved.   need to swap
with the temp page in new_req.    Fixed version in #for-next.

Thanks,
Miklos