Re: [PATCH 1/2] scsi: sd: fix write_same(16/10) to enable sector size > PAGE_SIZE

[Pankaj Raghav]

On 12/10/25 07:22, Damien Le Moal wrote:
> On 2025/12/09 17:41, sw.prabhu6@xxxxxxxxx wrote:
>> From: Swarna Prabhu <sw.prabhu6@xxxxxxxxx>
>>
>> The WRITE SAME(16) and WRITE SAME(10) scsi commands uses
>> a page from a dedicated mempool('sd_page_pool') for its
>> payload. This pool was initialized to allocate single
>> pages, which was sufficient as long as the device sector
>> size did not exceed the PAGE_SIZE.
>>
>> Given that block layer now supports block size upto
>> 64K ie beyond PAGE_SIZE, adapt sd_set_special_bvec()
>> to accommodate that.
>>
>> With the above fix, enable sector sizes > PAGE_SIZE in
>> scsi sd driver.
>>
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Swarna Prabhu <s.prabhu@xxxxxxxxxxx>
>> Co-developed-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
>> Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
>> ---
>> Note: We are allocating pages of order aligned to 
>> BLK_MAX_BLOCK_SIZE for the mempool page allocator
>> 'sd_page_pool' all the time. This is because we only
>> know that a bigger sector size device is attached at
>> sd_probe and it might be too late to reallocate mempool
>> with order >0.
> 
> That is a lot heavier on the memory for the vast majority of devices which are
> 512B or 4K block size... It may be better to have the special "large block"
> mempool attached to the scsi disk struct and keep the default single page
> mempool for all other regular devices.
> 

We had the same feeling as well and we mentioned it in the 1st RFC.

But when will you initialize the mempool for the large block devices? I don't think it
makes sense to unconditionally initialize it in init_sd.
Do we do it during the sd_probe() when we first encounter a large block device? That way
we may not waste any memory if no large block devices are attached.

--
Pankaj