Re: [PATCH] 9pfs: fix wrong I/O block size in Rgetattr

[Greg Kurz]

On Wed, 22 Sep 2021 17:55:02 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Mittwoch, 22. September 2021 17:42:08 CEST Philippe Mathieu-Daudé wrote:
> > On 9/22/21 15:13, Christian Schoenebeck wrote:
> > > When client sent a 9p Tgetattr request then the wrong I/O block
> > > size value was returned by 9p server; instead of host file
> > > system's I/O block size it should rather return an I/O block
> > > size according to 9p session's 'msize' value, because the value
> > > returned to client should be an "optimum" block size for I/O
> > > (i.e. to maximize performance), it should not reflect the actual
> > > physical block size of the underlying storage media.
> > > 
> > > The I/O block size of a host filesystem is typically 4k, so the
> > > value returned was far too low for good 9p I/O performance.
> > > 
> > > This patch adds stat_to_iounit() with a similar approach as the
> > > existing get_iounit() function.
> > > 
> > > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > > ---
> > > 
> > >   hw/9pfs/9p.c | 21 ++++++++++++++++++++-
> > >   1 file changed, 20 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> > > index c857b31321..708b030474 100644
> > > --- a/hw/9pfs/9p.c
> > > +++ b/hw/9pfs/9p.c
> > > @@ -1262,6 +1262,25 @@ static int coroutine_fn stat_to_v9stat(V9fsPDU
> > > *pdu, V9fsPath *path,> 
> > >   #define P9_STATS_ALL           0x00003fffULL /* Mask for All fields
> > >   above */> 
> > > +static int32_t stat_to_iounit(const V9fsPDU *pdu, const struct stat
> > > *stbuf) +{
> > > +    int32_t iounit = 0;
> > > +    V9fsState *s = pdu->s;
> > > +
> > > +    /*
> > > +     * iounit should be multiples of st_blksize (host filesystem block
> > > size) +     * as well as less than (client msize - P9_IOHDRSZ)
> > > +     */
> > > +    if (stbuf->st_blksize) {
> > > +        iounit = stbuf->st_blksize;
> > > +        iounit *= (s->msize - P9_IOHDRSZ) / stbuf->st_blksize;
> > 
> > Is that:
> > 
> >    iounit = QEMU_ALIGN_DOWN(s->msize - P9_IOHDRSZ, stbuf->st_blksize);
> > 
> > ?
> > 
> 
> Yes it is, thanks for the hint! :)
> 
> I actually just took the equivalent, already existing code from get_iounit():
> https://github.com/qemu/qemu/blob/2c3e83f92d93fbab071b8a96b8ab769b01902475/hw/9pfs/9p.c#L1880
> 
> Would it be OK to do that subsequently with cleanup patches? My plan was to
> first address this with one patch, and addressing the cleanup issues
> separately later on, because this patch is required for testing the following
> kernel patches:
> https://lore.kernel.org/netdev/cover.1632156835.git.linux_oss@crudebyte.com/
> 
> And I wanted to keep things simple by only requiring one patch on QEMU side
> for now.
> 

Fair enough and you're the maintainer anyway so this is your
call. :-)

Subsequent cleanup would be to switch to QEMU_ALIGN_DOWN() like
Philippe suggested but also to consolidate the logic in a common
helper in order to avoid the code duplication.

The patch is correct and simple enough to be merged as is :

Reviewed-by: Greg Kurz <groug@kaod.org>

> 
> > > +    }
> > > +    if (!iounit) {
> > > +        iounit = s->msize - P9_IOHDRSZ;
> > > +    }
> > > +    return iounit;
> > > +}
> > > +
> > > 
> > >   static int stat_to_v9stat_dotl(V9fsPDU *pdu, const struct stat *stbuf,
> > >   
> > >                                   V9fsStatDotl *v9lstat)
> > >   
> > >   {
> > > 
> > > @@ -1273,7 +1292,7 @@ static int stat_to_v9stat_dotl(V9fsPDU *pdu, const
> > > struct stat *stbuf,> 
> > >       v9lstat->st_gid = stbuf->st_gid;
> > >       v9lstat->st_rdev = stbuf->st_rdev;
> > >       v9lstat->st_size = stbuf->st_size;
> > > 
> > > -    v9lstat->st_blksize = stbuf->st_blksize;
> > > +    v9lstat->st_blksize = stat_to_iounit(pdu, stbuf);
> > > 
> > >       v9lstat->st_blocks = stbuf->st_blocks;
> > >       v9lstat->st_atime_sec = stbuf->st_atime;
> > >       v9lstat->st_atime_nsec = stbuf->st_atim.tv_nsec;
> 
>