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Date: Wed, 18 Mar 2026 11:53:26 -0600
From: Keith Busch <kbusch@kernel.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Keith Busch <kbusch@meta.com>, dm-devel@lists.linux.dev,
	linux-block@vger.kernel.org, snitzer@kernel.org
Subject: Re: [RESEND PATCHv3 2/2] dm-crypt: dynamic scatterlist for many
 segments
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References: <20260316150229.1771884-1-kbusch@meta.com>
 <20260316150941.1813568-1-kbusch@meta.com>
 <08fbfa54-0e19-82b5-01ba-216a4202d2e5@redhat.com>
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In-Reply-To: <7cdd50b4-afcf-aa5a-af0b-a00a35b88bdf@redhat.com>

On Wed, Mar 18, 2026 at 06:40:52PM +0100, Mikulas Patocka wrote:
> On Wed, 18 Mar 2026, Keith Busch wrote:
> 
> > But even then, the only thing I know of that really wants this has an
> > offset that straddles two pages per block, so I never need more than 2
> > segments, and the inline scatterlist has four. There's just currently no
> > way for the block layer to report a max-segments-per-block limit, so I'm
> > including this patch to be consistent with the reportable limits.
> 
> Can userspace use preadv/pwritev with many small iovecs on a file opened 
> for direct I/O?

Yes, I have several patches that were accepted last year to block and
iomap that align direct-io memory requirements to the backing device's
dma constraints. You can do dword aligned and sized vectored IO to a
block device today if the block limits say its okay.