Re: [PATCH v3 1/7] file-posix: fix max_iov for /dev/sg devices

[Vladimir Sementsov-Ogievskiy]

07.06.2021 08:39, Vladimir Sementsov-Ogievskiy wrote:
> 03.06.2021 16:37, Paolo Bonzini wrote:
> > Even though it was only called for devices that have bs->sg set (which
> > must be character devices),
> > sg_get_max_segments looked at /sys/dev/block which only works for
> > block devices.
> >
> > On Linux the sg driver has its own way to provide the maximum number of
> > iovecs in a scatter/gather list.
> >
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >   block/file-posix.c | 11 +++++++++++
> >   1 file changed, 11 insertions(+)
> >
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index f37dfc10b3..58db526cc2 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -1180,6 +1180,17 @@ static int sg_get_max_segments(int fd)
> >           goto out;
> >       }
> > +    if (S_ISCHR(st->st_mode)) {
> > +        if (ioctl(fd, SG_GET_SG_TABLESIZE, &ret) == 0) {
> > +            return ret;
>
> So, this is a new most-probable path for char-dev, yes?
>
> Is it better do it at start of the function to avoid extra fstat() ?

Haha, stupid question, sorry my morning sleepy eyes. fstat is used for outer if 
condition.

> > +        }
> > +        return -EIO;
> > +    }
> > +
> > +    if (!S_ISBLK(st->st_mode)) {
> > +        return -ENOTSUP;
> > +    }
> > +
> >       sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
> >                                   major(st.st_rdev), minor(st.st_rdev));
> >       sysfd = open(sysfspath, O_RDONLY);


--
Best regards,
Vladimir