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
next prev parent 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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