Re: [RFC][PATCH] kmap_atomic_push

[Peter Zijlstra <peterz@infradead.org>]

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.