Subject: Re: Integration of SCST in the mainstream Linux kernel
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Vladislav Bolkhovitin <vst@vlnb.net>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>,
       linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
       scst-devel@lists.sourceforge.net,
       Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@linux-foundation.org>
In-Reply-To: <47A8B210.8040202@vlnb.net>
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Date: Tue, 05 Feb 2008 13:13:22 -0600
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On Tue, 2008-02-05 at 21:59 +0300, Vladislav Bolkhovitin wrote:
> >>Hmm, how can one write to an mmaped page and don't touch it?
> > 
> > I meant from user space ... the writes are done inside the kernel.
> 
> Sure, the mmap() approach agreed to be unpractical, but could you 
> elaborate more on this anyway, please? I'm just curious. Do you think 
> about implementing a new syscall, which would put pages with data in the 
> mmap'ed area?

No, it has to do with the way invalidation occurs.  When you mmap a
region from a device or file, the kernel places page translations for
that region into your vm_area.  The regions themselves aren't backed
until faulted.  For write (i.e. incoming command to target) you specify
the write flag and send the area off to receive the data.  The gather,
expecting the pages to be overwritten, backs them with pages marked
dirty but doesn't fault in the contents (unless it already exists in the
page cache).  The kernel writes the data to the pages and the dirty
pages go back to the user.  msync() flushes them to the device.

The disadvantage of all this is that the handle for the I/O if you will
is a virtual address in a user process that doesn't actually care to see
the data. non-x86 architectures will do flushes/invalidates on this
address space as the I/O occurs.


> > However, as Linus has pointed out, this discussion is getting a bit off
> > topic. 
> 
> No, that isn't off topic. We've just proved that there is no good way to 
> implement zero-copy cached I/O for STGT. I see the only practical way 
> for that, proposed by FUJITA Tomonori some time ago: duplicating Linux 
> page cache in the user space. But will you like it?

Well, there's no real evidence that zero copy or lack of it is a problem
yet.

James


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