Re: [PATCH v7 16/21] block: add check when merging zone device pages

Logan Gunthorpe
2022-06-29T16:06:20Z

On 2022-06-29 00:46, Christoph Hellwig wrote:
> On Wed, Jun 15, 2022 at 10:12:28AM -0600, Logan Gunthorpe wrote:
>> Consecutive zone device pages should not be merged into the same sgl
>> or bvec segment with other types of pages or if they belong to different
>> pgmaps. Otherwise getting the pgmap of a given segment is not possible
>> without scanning the entire segment. This helper returns true either if
>> both pages are not zone device pages or both pages are zone device
>> pages with the same pgmap.
>>
>> Add a helper to determine if zone device pages are mergeable and use
>> this helper in page_is_mergeable().
>
> Any reason not to simply set REQ_NOMERGE for these requests? We
> can't merge for passthrough requests anyway, and genrally don't merge
> for direct I/O either, so adding all this overhead seems a bit pointless.

Hmm, I suppose we could also ensure that REQ_NOMERGE is set in a bio
before setting FOLL_PCI_P2PDMA in bio_map_user_iov() and
__bio_iov_iter_get_pages(). Assuming it's always set for any direct I/O.

I'll look into it.

Logan