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    SubjectRe: [PATCH v2 4/4] splice: fix updating sd->pos wrongly
    Changli Gao wrote:
    > On Fri, May 28, 2010 at 6:13 PM, Miklos Szeredi <miklos@szeredi.hu> wrote:
    > > On Fri, 28 May 2010, Changli Gao wrote:
    > >> On Fri, May 28, 2010 at 5:42 PM, Miklos Szeredi <miklos@szeredi.hu> wrote:
    > >> > On Wed, 26 May 2010, Changli Gao wrote:
    > >> >> fix updating sd->pos wrongly.
    > >> >>
    > >> >> In error path, we don't need to updating sd->pos, if the file isn't seekable.
    > >> >
    > >> > This patch is nonsense.  Why should we handle sd->pos != 0 case
    > >> > differently?
    > >> >
    > >>
    > >> If the in file isn't seekable, its splice_read won't update *ppos, so
    > >> in the error path, we'd better not change it too. Otherwise, some
    > >> assumption will go wrong.
    > >
    > > That may be true, but the patch is still nonsense.
    > >
    > > Look, your patch is updating/not updating sd->pos based on whether it
    > > is zero or not.  It will prevent updating the position for sockets,
    > > but it will also prevent updating the position for regular files if
    > > the position is zero, which is really not what we want.
    >
    > I think you misread my patch. Before checking the sd->pos, sd->pos
    > already is updated with the value returned by splice_read(), so if in
    > file is seekabble, sd->pos is non-zero when I checking it.

    Not true if the "file" is /dev/mem or /dev/kmem. The starting offset
    can be negative, so can end at zero.

    It's an obscure case and I don't know if you can sendfile from
    /dev/{,k}mem anyway :-) but illustrates why sd->pos != 0 is dubious.

    -- Jamie
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