Re: [Qemu-devel] [PATCH 1/2] glusterfs: fix max_discard

[Peter Lieven]

Am 03.02.2015 um 08:31 schrieb Denis V. Lunev:
> On 02/02/15 23:46, Denis V. Lunev wrote:
>> On 02/02/15 23:40, Peter Lieven wrote:
>>> Am 02.02.2015 um 21:09 schrieb Denis V. Lunev:
>>>> qemu_gluster_co_discard calculates size to discard as follows
>>>>      size_t size = nb_sectors * BDRV_SECTOR_SIZE;
>>>>      ret = glfs_discard_async(s->fd, offset, size, &gluster_finish_aiocb, 
>>>> acb);
>>>>
>>>> glfs_discard_async is declared as follows:
>>>>    int glfs_discard_async (glfs_fd_t *fd, off_t length, size_t lent,
>>>>                            glfs_io_cbk fn, void *data) __THROW
>>>> This is problematic on i686 as sizeof(size_t) == 4.
>>>>
>>>> Set bl_max_discard to SIZE_MAX >> BDRV_SECTOR_BITS to avoid overflow
>>>> on i386.
>>>>
>>>> Signed-off-by: Denis V. Lunev <address@hidden>
>>>> CC: Kevin Wolf <address@hidden>
>>>> CC: Peter Lieven <address@hidden>
>>>> ---
>>>>   block/gluster.c | 9 +++++++++
>>>>   1 file changed, 9 insertions(+)
>>>>
>>>> diff --git a/block/gluster.c b/block/gluster.c
>>>> index 1eb3a8c..8a8c153 100644
>>>> --- a/block/gluster.c
>>>> +++ b/block/gluster.c
>>>> @@ -622,6 +622,11 @@ out:
>>>>       return ret;
>>>>   }
>>>>   +static void qemu_gluster_refresh_limits(BlockDriverState *bs, Error 
>>>> **errp)
>>>> +{
>>>> +    bs->bl.max_discard = MIN(SIZE_MAX >> BDRV_SECTOR_BITS, INT_MAX);
>>>> +}
>>>> +
>>> Looking at the gluster code bl.max_transfer_length should have the same 
>>> limit, but thats a different patch.
>> ha, the same applies to nbd code too.
>>
>> I'll do this stuff tomorrow and also I think that some
>> audit in other drivers could reveal something interesting.
>>
>> Den
> ok. The situation is well rotten here on i686.
>
> The problem comes from the fact that QEMUIOVector
> and iovec uses size_t as length. All API calls use
> this abstraction. Thus all conversion operations
> from nr_sectors to size could bang at any moment.
>
> Putting dirty hands here is problematic from my point
> of view. Should we really care about this? 32bit
> applications are becoming old good history of IT...

The host has to be 32bit to be in trouble. And at least if we have KVM the host
has to support long mode.

I have on my todo to add generic code for honouring bl.max_transfer_length
in block.c. We could change default maximum from INT_MAX to SIZE_MAX >> 
BDRV_SECTOR_BITS
for bl.max_transfer_length.

Peter