Re: [PATCH] block: fix splitting segments

Guenter Roeck
2020-01-10T12:48:53Z

On 1/9/20 11:15 PM, Ming Lei wrote:
> On Fri, Jan 10, 2020 at 02:37:44PM +0800, 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.
>>
>>>
>>> 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.
>
> Or could you test the following patch?
>

That patch applied to ToT (without the other patch) fixes the problem for me.

Tested-by: Guenter Roeck <linux@roeck-us.net>

Guenter

> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 347782a24a35..d1bed8c382bf 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -164,8 +164,13 @@ static inline unsigned get_max_segment_size(const struct request_queue *q,
> unsigned long mask = queue_segment_boundary(q);
>
> offset = mask & (page_to_phys(start_page) + offset);
> - return min_t(unsigned long, mask - offset + 1,
> - queue_max_segment_size(q));
> +
> + /*
> + * overflow may be triggered in case of zero page physical address
> + * on 32bit arch, use queue's max segment size when that happens
> + */
> + return min_not_zero(mask - offset + 1,
> + (unsigned long)queue_max_segment_size(q));
> }
>
> /**
>
>
> Thanks,
> Ming
>