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Subject: Re: [PATCH 2/2] fuse: Replace page without copying in
 fuse_writepage_in_flight()
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <30c6d750-01b1-fb7e-eb80-0f60f242974a@virtuozzo.com>
Date:   Tue, 15 Jan 2019 19:14:39 +0300
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On 15.01.2019 18:39, Miklos Szeredi wrote:
> 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.

You are sure, page is just a simple pointer, not struct **page.
Then we would have had to change fuse_writepage_in_flight() to use ** pointer.

Updated version looks good for me.

Thanks,
Kirill