Re: [PATCH] block: fix splitting segments

Guenter Roeck
2020-01-10T05:10:28Z

On 1/9/20 7:00 PM, Ming Lei wrote:
> On Fri, Jan 10, 2020 at 10:58:01AM +0800, Ming Lei wrote:
>> On Thu, Jan 09, 2020 at 08:18:04AM -0700, Jens Axboe wrote:
>>> On 1/9/20 12:16 AM, Christoph Hellwig wrote:
>>>> On Thu, Jan 09, 2020 at 10:03:41AM +0800, Ming Lei wrote:
>>>>> It has been addressed in:
>>>>>
>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git/commit/?h=block-5.5&id=ecd255974caa45901d0b8fab03626e0a18fbc81a
>>>>
>>>> That is probably correct, but still highly suboptimal for most 32-bit
>>>> architectures where physical addresses are 32 bits wide. To fix that
>>>> the proper phys_addr_t type should be used.
>>>
>>> I'll swap it for phys_addr_t - we used to use dma_address_t or something
>>> like that, but I missed this type.
>>
>> Guenter mentioned that 'page_to_phys(start_page) as well as offset are
>> sometimes 0'[1].
>>
>> If that(zero page physical address) can happen when phys_addr_t is 32bit,
>> I guess phys_addr_t may not work too.
>>
>> Guener, could you test the patch in link[2] again?
>>
>>
>> [1] https://lore.kernel.org/linux-block/20200108023822.GB28075@ming.t460p/T/#m5862216b960454fc41a85204defbb887983bfd75
>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git/commit/?h=block-5.5&id=b6a89c4a9590663f80486662fe9a9c1f4cee31f4
>
> Loop Guener in.
>

The patch at [2] doesn't work.

My understanding is that the page in question is not mapped when
get_max_segment_size() is called (after all, the operation is the
result of a page fault). This is why page_to_phys() returns 0.

You'll either need a local u64 variable, or use some other means
to handle that situation. Something like

phys_addr_t paddr = page_to_phys(start_page);

if (paddr == 0)
return queue_max_segment_size(q);

at the beginning of the function might do, though there might
still be a problem when the page is later assigned and crosses
a segment boundary (if that is possible).

Guenter