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Date: Fri, 13 Jun 2025 08:19:12 +0200
From: Christoph Hellwig <hch@lst.de>
To: Kanchan Joshi <joshi.k@samsung.com>
Cc: Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	Keith Busch <kbusch@kernel.org>, Sagi Grimberg <sagi@grimberg.me>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Leon Romanovsky <leon@kernel.org>,
	Nitesh Shetty <nj.shetty@samsung.com>,
	Logan Gunthorpe <logang@deltatee.com>, linux-block@vger.kernel.org,
	linux-nvme@lists.infradead.org
Subject: Re: [PATCH 1/9] block: don't merge different kinds of P2P
 transfers in a single bio
Message-ID: <20250613061912.GA9517@lst.de>
References: <20250610050713.2046316-1-hch@lst.de> <CGME20250610050728epcas5p343c293cd81ffb03b6f12dabfcf2fd8d4@epcas5p3.samsung.com> <20250610050713.2046316-2-hch@lst.de> <c75ac878-376e-497b-b4fe-ddd54733c090@samsung.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Thu, Jun 12, 2025 at 11:54:21AM +0530, Kanchan Joshi wrote:
> On 6/10/2025 10:36 AM, Christoph Hellwig wrote:
> > --- a/block/bio.c
> > +++ b/block/bio.c
> > @@ -930,8 +930,6 @@ static bool bvec_try_merge_page(struct bio_vec *bv, struct page *page,
> >   		return false;
> >   	if (xen_domain() && !xen_biovec_phys_mergeable(bv, page))
> >   		return false;
> > -	if (!zone_device_pages_have_same_pgmap(bv->bv_page, page))
> > -		return false;
> 
> I did not understand the value of moving this out to its two callers 
> (bio_add_page and bio_integrity_add_page).
> 
> Since this check existed, I am a bit confused. The thing that patch 
> title says - is not a new addition and used to happen earlier too?
> Or is this about setting REQ_NOMERGE in bio?

It is about not merging mismatch pgmaps into a bio, for which it needs
to be in the caller.  The current code only prevents merges into the
same bio_vec.