Re: [Qemu-devel] [PATCH 4/4] block: avoid creating oversized writes in multiwrite_merge

[Kevin Wolf <kwolf@redhat.com>]

Am 23.09.2014 um 11:52 hat Peter Lieven geschrieben:
> On 23.09.2014 11:47, Kevin Wolf wrote:
> >Am 23.09.2014 um 11:32 hat Peter Lieven geschrieben:
> >>On 23.09.2014 10:59, Kevin Wolf wrote:
> >>>Am 23.09.2014 um 08:15 hat Peter Lieven geschrieben:
> >>>>On 22.09.2014 21:06, Paolo Bonzini wrote:
> >>>>>Il 22/09/2014 11:43, Peter Lieven ha scritto:
> >>>>>>This series aims not at touching default behaviour. The default for max_transfer_length
> >>>>>>is 0 (no limit). max_transfer_length is a limit that MUST be satisfied otherwise the request
> >>>>>>will fail. And Patch 2 aims at catching this fail earlier in the stack.
> >>>>>Understood.  But the right fix is to avoid that backend limits transpire
> >>>>>into guest ABI, not to catch the limits earlier.  So the right fix would
> >>>>>be to implement request splitting.
> >>>>Since you proposed to add traces for this would you leave those in?
> >>>>And since iSCSI is the only user of this at the moment would you
> >>>>go for implementing this check in the iSCSI block layer?
> >>>>
> >>>>As for the split logic would you think it is enough to build new qiov's
> >>>>out of the too big iov without copying the contents? This would work
> >>>>as long as a single iov inside the qiov is not bigger the max_transfer_length.
> >>>>Otherwise I would need to allocate temporary buffers and copy around.
> >>>You can split single iovs, too. There are functions that make this very
> >>>easy, they copy a sub-qiov with a byte granularity offset and length
> >>>(qemu_iovec_concat and friends). qcow2 uses the same to split requests
> >>>at (fragmented) cluster boundaries.
> >>Might it be as easy as this?
> >This is completely untested, right? :-)
> 
> Yes :-)
> I was just unsure if the general approach is right.

Looks alright to me.

> >But ignoring bugs, the principle looks right.
> >
> >>static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs,
> >>     int64_t sector_num, int nb_sectors, QEMUIOVector *qiov,
> >>     BdrvRequestFlags flags)
> >>{
> >>     if (nb_sectors < 0 || nb_sectors > (UINT_MAX >> BDRV_SECTOR_BITS)) {
> >>         return -EINVAL;
> >>     }
> >>
> >>     if (bs->bl.max_transfer_length &&
> >>         nb_sectors > bs->bl.max_transfer_length) {
> >>         int ret = 0;
> >>         QEMUIOVector *qiov2 = NULL;
> >Make it "QEMUIOVector qiov2;" on the stack.
> >
> >>         size_t soffset = 0;
> >>
> >>         trace_bdrv_co_do_readv_toobig(bs, sector_num, nb_sectors,
> >>bs->bl.max_transfer_length);
> >>
> >>         qemu_iovec_init(qiov2, qiov->niov);
> >And &qiov2 here, then this doesn't crash with a NULL dereference.
> >
> >>         while (nb_sectors > bs->bl.max_transfer_length && !ret) {
> >>             qemu_iovec_reset(qiov2);
> >>             qemu_iovec_concat(qiov2, qiov, soffset,
> >>                               bs->bl.max_transfer_length << BDRV_SECTOR_BITS);
> >>             ret = bdrv_co_do_preadv(bs, sector_num << BDRV_SECTOR_BITS,
> >>                                     bs->bl.max_transfer_length << BDRV_SECTOR_BITS,
> >>                                     qiov2, flags);
> >>             soffset += bs->bl.max_transfer_length << BDRV_SECTOR_BITS;
> >>             sector_num += bs->bl.max_transfer_length;
> >>             nb_sectors -= bs->bl.max_transfer_length;
> >>         }
> >>         qemu_iovec_destroy(qiov2);
> >>         if (ret) {
> >>             return ret;
> >>         }
> >The error check needs to be immediately after the assignment of ret,
> >otherwise the next loop iteration can overwrite an error with a success
> >(and if it didn't, it would still do useless I/O because the request as
> >a whole would fail anyway).
> 
> There is a && !ret in the loop condition. I wanted to avoid copying the destroy part.

Ah, yes, clever. I missed that. Maybe too clever then. ;-)

> BTW, is it !ret or ret < 0 ?

It only ever returns 0 or negative, so both are equivalent. I
prefer checks for ret < 0, but that's a matter of style rather than
correctness.

Another problem I just noticed is that this is not the only caller of
bdrv_co_do_preadv(), so you're not splitting all requests. The
synchronous bdrv_read/write/pread/pwrite/pwritev functions all don't get
the functionality this way.

Perhaps you should be doing it inside bdrv_co_do_preadv(), before the
call to bdrv_aligned_preadv(). Might even be more correct if it can
happen that the alignment adjustment increases a request too much to fit
in bl.max_transfer_length.

Kevin