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Subject: Re: [RFC][PATCH] kmap_atomic_push
From: Peter Zijlstra <peterz@infradead.org>
To: jim owens <jowens@hp.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Ingo Molnar <mingo@elte.hu>, Avi Kivity <avi@redhat.com>,
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Date: Fri, 09 Oct 2009 00:57:16 +0200
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On Thu, 2009-10-08 at 18:27 -0400, jim owens wrote:
> So if I understand this correctly, the sequence:
> 
> in = kmap_atomic(inpage, KM_USER1);
> 
> out = kmap_atomic(outpage, KM_USER0);
> 
> kunmap_atomic(in, KM_USER1);
> 
> in = kmap_atomic(next_inpage, KM_USER1);
> 
> is now illegal with this patch, which breaks code
> I am testing now for btrfs.
> 
> My code does this because the in/out are zlib inflate
> and the in/out run at different rates.

You can do things like:

do {
 in = kmap_atomic(inpage);
 out = kmap_atomic(outpage);

 <deflate until end of either in/out>

 kunmap_atomic(outpage);
 kunmap_atomic(inpage);

 cond_resched();

 <iterate bits>

} while (<not done>)

The double unmap gives a preemption point, which sounds like a good
thing to have, because your scheme could run for a long while without
enabling preemption, which is badness.

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