linux-kernel - Re: [PATCH 2/7] Define new syscalls readv2,preadv2,writev2,pwritev2

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Date:	Wed, 17 Sep 2014 12:59:30 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Milosz Tanski <milosz@...in.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	linux-aio@...ck.org, Mel Gorman <mgorman@...e.de>,
	Volker Lendecke <Volker.Lendecke@...net.de>,
	Tejun Heo <tj@...nel.org>, Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [PATCH 2/7] Define new syscalls readv2,preadv2,writev2,pwritev2

On Wed, Sep 17, 2014 at 12:05:23PM -0400, Milosz Tanski wrote:
> Theodore,
> 
> I might be missing understanding something, but... I already omitted
> read2 and write2 which can be implemented in userspace by libc (as you
> pointed out). In the case of readv vs. preadv there's an extra
> positional argument (file offset) and preadv version doesn't change
> the file location. I didn't want to overload the meaning of preadv2 to
> take a special negative offset value that uses the current file
> position but also changes the file position.

off_t has to be signed, so having a magic negative value doesn't
bother me that much.  Or you could use a flag bitvalue which means to
use the fd's offset and to ignore the positional value.  (More
bike-shedding :-)

The main reason why I mention it is we have a huge number of
read/write syscalls already, and if we add yet another to support
scatter-gather lists on the memory side, we'll be adding another
factor of two more read/write system calls.  So the suggestion was one
of trying to (probably fruitlessly) trying to stem the expnoential
increase in read/write system calls.  :-)

Cheers,

					- Ted
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