Re: [PATCH v4 1/7] file-posix: fix max_iov for /dev/sg devices

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

23.06.2021 18:42, Max Reitz wrote:
> On 08.06.21 21:14, Vladimir Sementsov-Ogievskiy wrote:
>> 08.06.2021 16:16, 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, so add support for it.  The block device
>>> path is kept because it will be reinstated in the next patches.
>>>
>>> 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..536998a1d6 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)) {
>>
>> Why not check "if (bs->sg) {" instead? It seems to be more consistent with issuing SG_ ioctl. Or what I miss?
> 
> I dismissed this in v3, because I didn’t understand why you’d raise this point.  The function is called sg_*(), and it’s only called if bs->sg is true anyway.  So clearly we can use SG_ ioctls, because the whole function is intended only for SG devices anyway.
> 
> This time, I looked forward, and perhaps starting at patch 4 I can understand where you’re coming from, because then the function is used for host devices in general.
> 
> So now I don’t particularly mind.  I think it’s still clear that if there’s a host device here that’s a character device, then that’s going to be an SG device, so I don’t really have a preference between S_ISCHR() and bs->sg.
> 

If I understand all correctly:

In this patch we don't need neither S_ISCHR nor bs->sg check: they both must pass for sg devices. Starting from patch 4 we'll need here if (bs->sg) check.

> 
>>> +        if (ioctl(fd, SG_GET_SG_TABLESIZE, &ret) == 0) {
>>> +            return ret;
>>> +        }
>>> +        return -ENOTSUP;
>>> +    }
>>> +
>>> +    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