Re: [PATCH V2] block: make segment size limit workable for > 4K PAGE_SIZE

[John Garry <john.g.garry@oracle.com>]

On 13/02/2025 09:58, Ming Lei wrote:
> On Thu, Feb 13, 2025 at 08:45:18AM +0000, John Garry wrote:
>> On 10/02/2025 09:03, Ming Lei wrote:
>>> PAGE_SIZE is applied in some block device queue limits, this way is
>>> very fragile and is wrong:
>>>
>>> - queue limits are read from hardware, which is often one readonly
>>> hardware property
>>>
>>> - PAGE_SIZE is one config option which can be changed during build time.
>>>
>>> In RH lab, it has been found that max segment size of some mmc card is
>>> less than 64K, then this kind of card can't work in case of 64K PAGE_SIZE.
>>>
>>> Fix this issue by using BLK_MIN_SEGMENT_SIZE in related code for dealing
>>> with queue limits and checking if bio needn't split. Define BLK_MIN_SEGMENT_SIZE
>>> as 4K(minimized PAGE_SIZE).
>> Please note that blk_queue_max_quaranteed_bio() for atomic writes assumes
>> that we can fit a PAGE_SIZE in a segment. I suppose that if the
>> max_segment_size < PAGE_SIZE is supported, then the calculation there needs
>> to change.
> It isn't related with blk_queue_max_guaranteed_bio() which calculates the max
> allowed ubuf bytes which can fit in a bio, so PAGE_SIZE has to be used here.
> 
> BLK_MIN_SEGMENT_SIZE is just one hint which can be used to check if one
> bvec can fit in single segment quickly, otherwise the normal split code
> path is run into.

So consider we have PAGE_SIZE > 4k and max_segment_size=4k, if an iovec 
has PAGE_SIZE then a bvec can also have PAGE_SIZE but then we need to 
split into multiple segments, right?

Thanks,
John