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From:   Christoph Hellwig <hch@lst.de>
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Subject: Re: [PATCH v7 16/21] block: add check when merging zone device
 pages
Message-ID: <20220629064629.GC17576@lst.de>
References: <20220615161233.17527-1-logang@deltatee.com> <20220615161233.17527-17-logang@deltatee.com>
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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.