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    SubjectRe: [PATCH 2/2] fuse: Replace page without copying in fuse_writepage_in_flight()
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    On 15.01.2019 19:36, Miklos Szeredi wrote:
    > On Tue, Jan 15, 2019 at 5:14 PM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
    >>
    >> 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.
    >
    > Using a struct page** would still have been broken, not because of
    > refcounting, but because of putting the wrong page into the request
    > (we do the temporary copy to avoid some issues with adding the page
    > cache page directly into the request)

    Ok, thanks for the explanation.

    Kirill

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