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

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Date:   Wed, 29 Jun 2022 08:46:29 +0200
From:   Christoph Hellwig <hch@....de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>,
        Ralph Campbell <rcampbell@...dia.com>
Subject: Re: [PATCH v7 16/21] block: add check when merging zone device
 pages

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.