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Date:   Wed, 29 Jun 2022 10:06:00 -0600
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To:     Christoph Hellwig <hch@lst.de>
Cc:     linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
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        Stephen Bates <sbates@raithlin.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Don Dutile <ddutile@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Minturn Dave B <dave.b.minturn@intel.com>,
        Jason Ekstrand <jason@jlekstrand.net>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Xiong Jianxin <jianxin.xiong@intel.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Martin Oliveira <martin.oliveira@eideticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@gmail.com>,
        Ralph Campbell <rcampbell@nvidia.com>
References: <20220615161233.17527-1-logang@deltatee.com>
 <20220615161233.17527-17-logang@deltatee.com> <20220629064629.GC17576@lst.de>
From:   Logan Gunthorpe <logang@deltatee.com>
In-Reply-To: <20220629064629.GC17576@lst.de>
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Subject: Re: [PATCH v7 16/21] block: add check when merging zone device pages
Precedence: bulk


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