linux-kernel - Re: No more bits in vm_area_struct's vm

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Message-ID: <4AC0234F.2080808@crca.org.au>
Date:	Mon, 28 Sep 2009 12:45:35 +1000
From:	Nigel Cunningham <ncunningham@...a.org.au>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: No more bits in vm_area_struct's vm_flags.

Hi.

KAMEZAWA Hiroyuki wrote:
> On Fri, 25 Sep 2009 18:34:56 +1000
> Nigel Cunningham <ncunningham@...a.org.au> wrote:
>> Hi.
>>
>> KAMEZAWA Hiroyuki wrote:
>>>> I have some code in TuxOnIce that needs a bit too (explicitly mark the
>>>> VMA as needing to be atomically copied, for GEM objects), and am not
>>>> sure what the canonical way to proceed is. Should a new unsigned long be
>>>> added? The difficulty I see with that is that my flag was used in
>>>> shmem_file_setup's flags parameter (drm_gem_object_alloc), so that
>>>> function would need an extra parameter too..
>>> Hmm, how about adding vma->vm_flags2 ?
>> The difficulty there is that some functions pass these flags as arguments.
>>
> Ah yes. But I wonder some special flags, which is rarey used, can be moved
> to vm_flags2...
> 
> For example,
> 
>  #define VM_SEQ_READ     0x00008000      /* App will access data sequentially */
>  #define VM_RAND_READ    0x00010000      /* App will not benefit from clustered reads */
> are all capsuled under
> mm.h
>  117 #define VM_READHINTMASK                 (VM_SEQ_READ | VM_RAND_READ)
>  118 #define VM_ClearReadHint(v)             (v)->vm_flags &= ~VM_READHINTMASK
>  119 #define VM_NormalReadHint(v)            (!((v)->vm_flags & VM_READHINTMASK))
>  120 #define VM_SequentialReadHint(v)        ((v)->vm_flags & VM_SEQ_READ)
>  121 #define VM_RandomReadHint(v)            ((v)->vm_flags & VM_RAND_READ)
> 
> Or
> 
> 105 #define VM_PFN_AT_MMAP  0x40000000      /* PFNMAP vma that is fully mapped at mmap time */
> is only used under special situation.
> 
> etc..
> 
> They'll be able to be moved to other(new) flag field, IIUC.

I'm working on a patch to do this, and am looking at is_mergeable_vma,
which is invoked via can_vma_merge_after from vma_merge from
madvise_behaviour (which potentially modifies these hint flags). Should
those hints be considered in that function? (Do I need to pass the hints
in as well and check they're equal?)

By the way, VM_ClearReadHint and VM_NormalReadHint are currently unused.
 madvise_behaviour manipulates the flags directly (in preparing
potential replacement values). Not sure if something should be done
about that.

By the way #2, in response to the later message in this thread, I'm
calling the new var vma->vm_hints, and have put it at the end of the
struct at the moment. Is that a good place?

Regards,

Nigel
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