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Date: Wed, 27 Aug 2025 09:37:09 +0200
From: Christoph Hellwig <hch@lst.de>
To: Keith Busch <kbusch@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Christoph Hellwig <hch@infradead.org>,
	Keith Busch <kbusch@meta.com>, linux-block@vger.kernel.org,
	linux-nvme@lists.infradead.org, axboe@kernel.dk,
	iommu@lists.linux.dev
Subject: Re: [PATCHv3 1/2] block: accumulate segment page gaps per bio
Message-ID: <20250827073709.GA25032@lst.de>
References: <20250821204420.2267923-1-kbusch@meta.com> <20250821204420.2267923-2-kbusch@meta.com> <aKxpSorluMXgOFEI@infradead.org> <aKxu83upEBhf5gT7@kbusch-mbp> <20250826130344.GA32739@lst.de> <aK27AhpcQOWADLO8@kbusch-mbp> <20250826135734.GA4532@lst.de> <aK42K_-gHrOQsNyv@kbusch-mbp>
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On Tue, Aug 26, 2025 at 04:33:15PM -0600, Keith Busch wrote:
> On Tue, Aug 26, 2025 at 03:57:34PM +0200, Christoph Hellwig wrote:
> > On Tue, Aug 26, 2025 at 07:47:46AM -0600, Keith Busch wrote:
> > > Currently, the virtual boundary is always compared to bv_offset, which
> > > is a page offset. If the virtual boundary is larger than a page, then we
> > > need something like "page_to_phys(bv.bv_page) + bv.bv_offset" every
> > > place we need to check against the virt boundary.
> > 
> > bv_offset is only guaranteed to be a page offset if your use
> > bio_for_each_segment(_all) or the low-level helpers implementing
> > it and not bio_for_each_bvec(_all) where it can be much larger
> > than PAGE_SIZE.
> 
> Yes, good point. So we'd have a folio offset when it's not a single
> page, but I don't think we want to special case large folios for every
> virt boundary check. It's looking like replace bvec's "page + offset"
> with phys addrs, yeah?!

Basically everything should be using physical address.  The page + offset
is just a weird and inefficient way to represent that and we really
need to get rid of it.