CMA region and highmem

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

Marek,

Is there a reason that CMA only steals memory from lowmem, and does
not allocate from highmem?

The reason I ask is that this morning, I was greeted by this excellent
OOM on one of my platforms.  It has 2G of memory, and no real DMA
restrictions, and with much of the memory still available, it OOM'd
because I need a relatively large CMA block.

If CMA were pushed into highmem, this OOM would not have happened.

Please don't mention anything suggesting that the reason it's in lowmem
is because of DMA-able memory restrictions - that's a totally bogus and
insane argument.  The highmem/lowmem split is purely a software thing
and has nothing to do with the hardware at all.  I'll illustrate:

 Take a platform with 2G of memory.  You configure it with PAGE_OFFSET
 at 3GB.  You end up with about 760MB of lowmem, the rest as highmem.
 Let's say (for the sake of argument) that you rely on that split,
 beacuse you have devices which don't work with the second 1GB of
 memory.

 Now I build that same kernel, but configure PAGE_OFFSET at 1GB.  Bingo,
 all 2G of memory is now mapped as lowmem, and there is nothing stopping
 any of the 2G of memory being handed out for DMA purposes.

Therefore, talking about highmem/lowmem in the same sentence as DMA-able
memory is totally wrong.

Anyway, back to the OOM:

xdg-screensaver invoked oom-killer: gfp_mask=0x4d0, order=2, oom_score_adj=0
CPU: 2 PID: 6476 Comm: xdg-screensaver Not tainted 3.16.0+ #1322
...
[<c001c8e8>] (pgd_alloc) from [<c00237b8>] (mm_init.isra.51+0xc0/0x120)
...
Mem-info:
Normal per-cpu:
CPU    0: hi:  186, btch:  31 usd:  29
CPU    1: hi:  186, btch:  31 usd:   0
CPU    2: hi:  186, btch:  31 usd:   0
CPU    3: hi:  186, btch:  31 usd:   0
HighMem per-cpu:
CPU    0: hi:  186, btch:  31 usd:   0
CPU    1: hi:  186, btch:  31 usd:  21
CPU    2: hi:  186, btch:  31 usd:   0
CPU    3: hi:  186, btch:  31 usd:   0
active_anon:47933 inactive_anon:2583 isolated_anon:0
 active_file:16171 inactive_file:71321 isolated_file:0
 unevictable:0 dirty:0 writeback:0 unstable:0
 free:244622 slab_reclaimable:3605 slab_unreclaimable:95994
 mapped:21360 shmem:5230 pagetables:1176 bounce:0
 free_cma:37697
Normal free:159400kB min:3440kB low:4300kB high:5160kB active_anon:54336kB
 inactive_anon:2580kB active_file:56kB inactive_file:204kB unevictable:0kB
 isolated(anon):0kB isolated(file):0kB present:778240kB managed:740044kB
 mlocked:0kB dirty:0kB writeback:0kB mapped:5336kB shmem:5428kB
 slab_reclaimable:14420kB slab_unreclaimable:383976kB kernel_stack:2512kB
 pagetables:1088kB unstable:0kB bounce:0kB free_cma:150788kB
 writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
lowmem_reserve[]: 0 10304 10304
HighMem free:818228kB min:512kB low:2044kB high:3576kB active_anon:138352kB
 inactive_anon:7752kB active_file:64692kB inactive_file:285204kB unevictable:0kB
 isolated(anon):0kB isolated(file):0kB present:1318912kB managed:1318912kB
 mlocked:0kB dirty:0kB writeback:0kB mapped:80072kB shmem:15492kB
 slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB
 pagetables:3636kB unstable:0kB bounce:0kB free_cma:0kB
 writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
lowmem_reserve[]: 0 0 0
Normal: 117*4kB (UEMC) 959*8kB (UEMC) 2*16kB (C) 2*32kB
 (C) 2*64kB (C) 0*128kB 0*256kB 0*512kB 1*1024kB (C) 1*2048kB (C)
 36*4096kB (C) = 158892kB
HighMem: 32*4kB (UM) 75*8kB (UM) 633*16kB (UM) 377*32kB (UM) 191*64kB
 (UM) 86*128kB (UM) 40*256kB (M) 19*512kB (M) 14*1024kB (UM) 6*2048kB
 (UM) 177*4096kB (MR) = 817736kB
92769 total pagecache pages
0 pages in swap cache
Swap cache stats: add 0, delete 0, find 0/0
Free swap  = 0kB
Total swap = 0kB
524288 pages of RAM
244139 free pages
9549 reserved pages
27079 slab pages
55230910 pages shared
0 pages swap cached
...
Out of memory: Kill process 3460 (vlc) score 38 or sacrifice child
Killed process 3460 (vlc) total-vm:278340kB, anon-rss:30360kB, file-rss:48988kB

As you can see, there's plenty of precious lowmem still available, but
because we only have 760MB of lowmem, from which we steal 256MB for
CMA, this leaves around 500MB of lowmem free - and that puts quite a
lot of pressure on lowmem.  Meanwhile, we have oodles of highmem still
available which would be better suited to CMA - especially as CMA would
not have to do dance around changing the page tables.

Any thoughts?

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