Re: [PATCH 3/3] mm/vmalloc: get rid of dirty bitmap inside vmap_block structure

[Joonsoo Kim]

2015-03-13 21:12 GMT+09:00 Roman Pen <r.peniaev@xxxxxxxxx>:
> In original implementation of vm_map_ram made by Nick Piggin there were two
> bitmaps:  alloc_map and dirty_map.  None of them were used as supposed to be:
> finding a suitable free hole for next allocation in block. vm_map_ram allocates
> space sequentially in block and on free call marks pages as dirty, so freed
> space can't be reused anymore.
>
> Actually would be very interesting to know the real meaning of those bitmaps,
> maybe implementation was incomplete, etc.
>
> But long time ago Zhang Yanfei removed alloc_map by these two commits:
>
>   mm/vmalloc.c: remove dead code in vb_alloc
>      3fcd76e8028e0be37b02a2002b4f56755daeda06
>   mm/vmalloc.c: remove alloc_map from vmap_block
>      b8e748b6c32999f221ea4786557b8e7e6c4e4e7a
>
> In current patch I replaced dirty_map with two range variables: dirty min and
> max.  These variables store minimum and maximum position of dirty space in a
> block, since we need only to know the dirty range, not exact position of dirty
> pages.
>
> Why it was made? Several reasons: at first glance it seems that vm_map_ram
> allocator concerns about fragmentation thus it uses bitmaps for finding free
> hole, but it is not true.  To avoid complexity seems it is better to use
> something simple, like min or max range values.  Secondly, code also becomes
> simpler, without iteration over bitmap, just comparing values in min and max
> macros.  Thirdly, bitmap occupies up to 1024 bits (4MB is a max size of a
> block).  Here I replaced the whole bitmap with two longs.
>
> Finally vm_unmap_aliases should be slightly faster and the whole vmap_block
> structure occupies less memory.
>
> Signed-off-by: Roman Pen <r.peniaev@xxxxxxxxx>
> Cc: Nick Piggin <npiggin@xxxxxxxxx>
> Cc: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: WANG Chao <chaowang@xxxxxxxxxx>
> Cc: Fabian Frederick <fabf@xxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Gioh Kim <gioh.kim@xxxxxxx>
> Cc: Rob Jones <rob.jones@xxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx

Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
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