Re: [RFC 2/4] ARM: dma-mapping: IOMMU allocates pages from poolwith GFP_ATOMIC

[Hiroshi Doyu]

Hi,

KyongHo Cho <pullip.cho@xxxxxxxxxxx> wrote @ Wed, 22 Aug 2012 14:47:00 +0200:

> 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.

*1: