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From: Douglas Gilbert <[email protected]>
To: James Bottomley <[email protected]>,
	Christoph Hellwig <[email protected]>,
	Pavel Begunkov <[email protected]>
Cc: Ming Lei <[email protected]>,
	[email protected], Jens Axboe <[email protected]>,
	Matthew Wilcox <[email protected]>,
	Johannes Weiner <[email protected]>,
	Alexander Viro <[email protected]>,
	"Darrick J . Wong" <[email protected]>,
	"Martin K . Petersen" <[email protected]>,
	Jonathan Corbet <[email protected]>,
	[email protected], [email protected],
	[email protected], [email protected],
	[email protected], [email protected],
	[email protected]
Subject: Re: [PATCH v1 0/6] no-copy bvec
Date: Wed, 23 Dec 2020 15:23:57 -0500	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <f06ece44a86eb9c8ef07bbd9f6f53342366b7751.camel@HansenPartnership.com>

On 2020-12-23 11:04 a.m., James Bottomley wrote:
> On Wed, 2020-12-23 at 15:51 +0000, Christoph Hellwig wrote:
>> On Wed, Dec 23, 2020 at 12:52:59PM +0000, Pavel Begunkov wrote:
>>> Can scatterlist have 0-len entries? Those are directly translated
>>> into bvecs, e.g. in nvme/target/io-cmd-file.c and
>>> target/target_core_file.c. I've audited most of others by this
>>> moment, they're fine.
>>
>> For block layer SGLs we should never see them, and for nvme neither.
>> I think the same is true for the SCSI target code, but please double
>> check.
> 
> Right, no-one ever wants to see a 0-len scatter list entry.  The reason
> is that every driver uses the sgl to program the device DMA engine in
> the way NVME does.  a 0 length sgl would be a dangerous corner case:
> some DMA engines would ignore it and others would go haywire, so if we
> ever let a 0 length list down into the driver, they'd have to
> understand the corner case behaviour of their DMA engine and filter it
> accordingly, which is why we disallow them in the upper levels, since
> they're effective nops anyway.

When using scatter gather lists at the far end (i.e. on the storage device)
the T10 examples (WRITE SCATTERED and POPULATE TOKEN in SBC-4) explicitly
allow the "number of logical blocks" in their sgl_s to be zero and state
that it is _not_ to be considered an error.

Doug Gilbert


  reply	other threads:[~2020-12-23 20:25 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-15  0:20 [PATCH v1 0/6] no-copy bvec Pavel Begunkov
2020-12-15  0:20 ` [PATCH v1 1/6] target/file: allocate the bvec array as part of struct target_core_file_cmd Pavel Begunkov
2020-12-15  0:20 ` [PATCH v1 2/6] iov_iter: optimise bvec iov_iter_advance() Pavel Begunkov
2020-12-15  9:37   ` David Laight
2020-12-15 11:23     ` Pavel Begunkov
2020-12-15 13:54       ` David Laight
2020-12-15 13:56         ` Pavel Begunkov
2020-12-22 14:03   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 3/6] bio: deduplicate adding a page into bio Pavel Begunkov
2020-12-22 14:04   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 4/6] block/psi: remove PSI annotations from direct IO Pavel Begunkov
2020-12-15  0:56   ` Dave Chinner
2020-12-15  1:03     ` Pavel Begunkov
2020-12-15  1:33       ` Dave Chinner
2020-12-15 11:41         ` Pavel Begunkov
2020-12-22 14:07   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 5/6] bio: add a helper calculating nr segments to alloc Pavel Begunkov
2020-12-15  1:00   ` Dave Chinner
2020-12-15  1:07     ` Pavel Begunkov
2020-12-15  1:09     ` Dave Chinner
2020-12-22 14:07   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 6/6] block/iomap: don't copy bvec for direct IO Pavel Begunkov
2020-12-15  1:09   ` Dave Chinner
2020-12-15  1:15     ` Pavel Begunkov
2020-12-22 14:15   ` Christoph Hellwig
2020-12-15  1:41 ` [PATCH v1 0/6] no-copy bvec Ming Lei
2020-12-15 11:14   ` Pavel Begunkov
2020-12-15 12:03     ` Ming Lei
2020-12-15 14:05       ` Pavel Begunkov
2020-12-22 14:11         ` Christoph Hellwig
2020-12-23 12:52           ` Pavel Begunkov
2020-12-23 15:51             ` Christoph Hellwig
2020-12-23 16:04               ` James Bottomley
2020-12-23 20:23                 ` Douglas Gilbert [this message]
2020-12-23 20:32                   ` Pavel Begunkov
2020-12-24  6:41                     ` Christoph Hellwig
2020-12-24 16:45                       ` Douglas Gilbert
2020-12-24 17:30                   ` James Bottomley

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