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Date: Wed, 29 Jul 2026 13:31:08 +0200
From: Christoph Hellwig <hch@lst.de>
To: Anuj Gupta/Anuj Gupta <anuj20.g@samsung.com>
Cc: Christoph Hellwig <hch@lst.de>, Pavel Begunkov <asml.silence@gmail.com>,
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Subject: Re: [PATCH v4 09/14] nvme-pci: add SGL support for the dmabuf path
Message-ID: <20260729113108.GA31120@lst.de>
References: <cover.1785274111.git.asml.silence@gmail.com> <5c2e9c3bdd3e87c0c9a6e4395ea199002b83fd36.1785274111.git.asml.silence@gmail.com> <CGME20260729072126epcas5p3ad4422a45f1274b71f15dec8bf487fcc@epcas5p3.samsung.com> <20260729072117.GI9534@lst.de> <81412691.21785320403945.JavaMail.epsvc@epcpadp1new>
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On Wed, Jul 29, 2026 at 03:47:23PM +0530, Anuj Gupta/Anuj Gupta wrote:
> > But I also don't understand what the use case for this function
> > is to start with.  struct sg_table tells us how many segments
> > exist on the DMA side in the nents member, which should be just
> > fine for the SGL threshold calculation.
> 
> sg_table->nents covers the entire exported buffer (<=1GiB), while a
> request only covers a subrange[bi_offset, bi_offset+payload). Using
> nents would overcount the request's segments.

Urgg, yes.

> >> +	if (!entries)
> >> +		return BLK_STS_IOERR;
> >> +	if (entries > NVME_MAX_SEGS)
> >> +		return BLK_STS_AGAIN;
> > 
> > Given that the block layer enforced data in rw/command and the
> > max_segments limit, why do we need the extra check here?
> 
> A dmabuf bio reports nsegs=1 (bio_split_io_at) to the block layer, so 
> max_segments isn't enforced against the SG entries actually spanned by 
> the request. Hence the explicit check.

We'll need to expose the actual nsegs to the block layer and split
based on that.  Otherwise I/O might work or fail based on the device
capabilities.