Re: [PATCH] block: fix splitting segments

Guenter Roeck
2020-01-10T12:36:10Z

On 1/9/20 10:37 PM, Ming Lei wrote:
> On Thu, Jan 09, 2020 at 09:10:24PM -0800, Guenter Roeck wrote:
>> 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.
>
> page_to_phys() supposes to return page's physical address, which
> should just depend on this machine's physical address space,
> not related with page mapping.
>
> I understand physical address 0 might be valid, such as the 1st
> page frame of ram.
>

Not sure if that happens here, but makes sense.

>>
>> 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).
>
> IMO, zero physical address case is the only corner case not
> covered by using 'phys_addr_t'. If phys_addr_t is 32bit, sum of
> page_to_phys(start_page) and offset shouldn't be >= 4G.
>

Yes, but that isn't what is calculated. What is calculated is
mask - offset + 1
where
offset = mask & (page_to_phys(start_page) + offset);

with mask == 0xffffffff, offset == 0, we get:
mask - offset + 1 = 0xffffffff - 0 + 1 = 0x100000000, which is > 4G.

Guenter