On Wed, Aug 22, 2012 at 03:36:48PM +0200, Hiroshi Doyu wrote:

Hi,

KyongHo Cho [off-list ref] wrote @ Wed, 22 Aug 2012 14:47:00 +0200:

quoted

vzalloc() call in __iommu_alloc_buffer() also causes BUG() in atomic context.

Right.

I've been thinking that kzalloc() may be enough here, since
vzalloc() was introduced to avoid allocation failure for big chunk of
memory, but I think that it's unlikely that the number of page array
can be so big. So I propose to drop vzalloc() here, and just simply to
use kzalloc only as below(*1).

For example, 

1920(H) x 1080(W) x 4(bytes) ~= 8MiB

For 8 MiB buffer,
  8(MiB) * 1024 = 8192(KiB)
  8192(KiB) / 4(KiB/page) = 2048 pages
  sizeof(struct page *) = 4 bytes
  2048(pages) * 4(bytes/page) = 8192(bytes) = 8(KiB)
  8(KiB) / 4(KiB/page) = 2 pages

If the above estimation is right(I hope;)), the necessary pages are
_at most_ 2 pages. If the system gets into the situation to fail to
allocate 2 contiguous pages, that's real the problem. I guess that
that kind of fragmentation problem would be solved with page migration
or something, especially nowadays devices are getting larger memories.

In atomic context, VM have no choice except relying on kswapd so
high order allocation can fail easily when memory fragementation
is high.

-- 
Kind regards,
Minchan Kim

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