[GEDI] [PATCH v2] block/gluster: correctly set max_pdiscard

[Fabian Ebner]

Am 12.05.22 um 18:05 schrieb Stefano Garzarella:
> On Thu, May 12, 2022 at 05:44:13PM +0200, Stefano Garzarella wrote:
>> On Thu, May 12, 2022 at 12:30:48PM +0200, Fabian Ebner wrote:
>>> On 64-bit platforms, SIZE_MAX is too large for max_pdiscard, which is
>>
>> The main problem is that SIZE_MAX for an int64_t is a negative value.
>>

Yes, I should've stated that directly.

>>> int64_t, and the following assertion would be triggered:
>>> qemu-system-x86_64: ../block/io.c:3166: bdrv_co_pdiscard: Assertion
>>> `max_pdiscard >= bs->bl.request_alignment' failed.
>>>
>>> Fixes: 0c8022876f ("block: use int64_t instead of int in driver
>>> discard handlers")
>>> Cc: qemu-stable at nongnu.org
>>> Signed-off-by: Fabian Ebner <f.ebner at proxmox.com>
>>> ---
>>> block/gluster.c | 4 ++--
>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/block/gluster.c b/block/gluster.c
>>> index 398976bc66..f711bf0bd6 100644
>>> --- a/block/gluster.c
>>> +++ b/block/gluster.c
>>> @@ -891,7 +891,7 @@ out:
>>> static void qemu_gluster_refresh_limits(BlockDriverState *bs, Error
>>> **errp)
>>> {
>>>    bs->bl.max_transfer = GLUSTER_MAX_TRANSFER;
>>> -    bs->bl.max_pdiscard = SIZE_MAX;
>>> +    bs->bl.max_pdiscard = MIN(SIZE_MAX, INT64_MAX);
>>
>> What would be the problem if we use INT64_MAX?
> 
> Okay, I just saw Eric's answer to v1 and I think this is right.
> 

Sorry for not mentioning the changes from v1.

> Please explain it in the commit message and also the initial problem
> that is SIZE_MAX on a 64-bit platform is a negative number for int64_t,
> so the assert fails.
> 

I'll try and improve the commit message for v3.

> Thanks,
> Stefano
> 
>> (I guess the intention of the original patch was to set the maximum
>> value in drivers that do not have a specific maximum).
>>
>> Or we can set to 0, since in block/io.c we have this code:
>>
>>    max_pdiscard = QEMU_ALIGN_DOWN(MIN_NON_ZERO(bs->bl.max_pdiscard,
>> INT64_MAX),
>>                                   align);
>>    assert(max_pdiscard >= bs->bl.request_alignment);
>>
>> Where `max_pdiscard` is set to INT64_MAX (and aligned) if
>> bs->bl.max_pdiscard is 0.
>>
>>> }
>>>
>>> static int qemu_gluster_reopen_prepare(BDRVReopenState *state,
>>> @@ -1304,7 +1304,7 @@ static coroutine_fn int
>>> qemu_gluster_co_pdiscard(BlockDriverState *bs,
>>>    GlusterAIOCB acb;
>>>    BDRVGlusterState *s = bs->opaque;
>>>
>>> -    assert(bytes <= SIZE_MAX); /* rely on max_pdiscard */
>>> +    assert(bytes <= MIN(SIZE_MAX, INT64_MAX)); /* rely on
>>> max_pdiscard */
>>
>> Can we use bs->bl.max_pdiscard directly here?
>>

Now I'm thinking that the assert is actually for checking that the value
can be passed to glfs_discard_async(), which takes a size_t for the
argument in question. So maybe it's best to keep assert(bytes <=
SIZE_MAX) as is?

>> Thanks,
>> Stefano
> 
> 
>