linux-kernel - Re: [PATCH 3/3] mm/vmalloc: get rid of dirty bitmap inside vmap

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Message-ID: <CAAmzW4MPkFzV0kuQy=nE9nWqFRWCq3S00_DgGiYtPtf2Lzp7_A@mail.gmail.com>
Date:	Wed, 18 Mar 2015 14:52:03 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Roman Pen <r.peniaev@...il.com>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	David Rientjes <rientjes@...gle.com>,
	WANG Chao <chaowang@...hat.com>,
	Fabian Frederick <fabf@...net.be>,
	Christoph Lameter <cl@...ux.com>, Gioh Kim <gioh.kim@....com>,
	Rob Jones <rob.jones@...ethink.co.uk>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm/vmalloc: get rid of dirty bitmap inside vmap_block structure

2015-03-13 21:12 GMT+09:00 Roman Pen <r.peniaev@...il.com>:
> 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@...il.com>
> Cc: Nick Piggin <npiggin@...nel.dk>
> Cc: Zhang Yanfei <zhangyanfei@...fujitsu.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: WANG Chao <chaowang@...hat.com>
> Cc: Fabian Frederick <fabf@...net.be>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Gioh Kim <gioh.kim@....com>
> Cc: Rob Jones <rob.jones@...ethink.co.uk>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org

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