Re: mmap dirty limits on 32 bit kernels (Was: [BUG] New Kernel Bugs)

[Bron Gondwana <brong@fastmail.fm>]

On Thu, Nov 15, 2007 at 08:32:22AM -0800, Linus Torvalds wrote:
> On Thu, 15 Nov 2007, Bron Gondwana wrote:
> > 
> > I guess we'll be doing the one-liner kernel mod and testing
> > that then.
> 
> The thing to look at is "get_dirty_limits()" in mm/page-writeback.c, and 
> in this particular case it's the
> 
> 	unsigned long available_memory = determine_dirtyable_memory();
> 
> that's going to bite you. In particular, note the
> 
> 	x -= highmem_dirtyable_memory(x);
> 
> that we do in determine_dirtyable_memory().
> 
> So in this case, if you basically remove that line, it will allow all of 
> memory to be dirtied (including highmem), and then the background_ratio 
> will work on the whole 6GB.
> 
> HOWEVER! It's worth noting that we also have some other old legacy cruft 
> there that may interfere with your code. In particular, if you look at the 
> top of "get_dirty_limits()", it *also* does a
> 
>         unmapped_ratio = 100 - ((global_page_state(NR_FILE_MAPPED) +
>                                 global_page_state(NR_ANON_PAGES)) * 100) /
>                                         available_memory;
> 
>         dirty_ratio = vm_dirty_ratio;
>         if (dirty_ratio > unmapped_ratio / 2)
>                 dirty_ratio = unmapped_ratio / 2;
> 
> and that whole "unmapped_ratio" comparison is probably bogus these days, 
> since we now take the mapped dirty pages into account. That code harks 
> back to the days before we did that, and dirty ratios only affected 
> non-mapped pages.
> 
> And in particular, now that I look at it, I wonder if it can even go 
> negative (because "available_memory" may be *smaller* than the 
> NR_FILE_MAPPED|ANON_PAGES sum!).
> 
> We'll fix up a negative value anyway (because of the clamping of 
> dirty_ratio to no less than 5), but the point is that the whole 
> "unmapped_ratio" thing probably doesn't make sense any more, and may well 
> make the dirty_ratio not work for you, because you may have a very small 
> unmapped_ratio that effectively makes all dirty limits always clamp to a 
> very small value.
> 
> So regardless, I think you may want to try the appended patch *first*.
> 
> If this patch makes a difference, please holler. I think it's the correct 
> thing to do, but I'm not going to actually commit it without somebody 
> saying that it makes a difference (and preferably Peter Zijlstra and 
> Andrew acking it too).

mmap: mmap call failed: errno: 12 errmsg: Cannot allocate memory

Yep, that's "fixed" the problem alright!  No way this puppy is
dirtying 2Gb of memory any more.

http://linux.brong.fastmail.fm/2007-11-22/bmtest.pl

That said, pushing the size down to 1700 rather than 2000 in that
file makes it run, and the behaviour matches the 2000 Mb case on
2.6.16.55 rather than 2.6.20.20 or 2.6.23.1 (my other test case
kernels that happened to be pre-built on that machine)

[root@lb1 ~]$ free
             total       used       free     shared    buffers     cached
Mem:       4149836    2073056    2076780          0      22036    1846096
-/+ buffers/cache:     204924    3944912
Swap:      2096472          0    2096472

That's after running the 1700Mb version.  You can see this machine is our
one remaining 4Gb machine (it's not running any production services unlike
the 6Gb machine, so it's better for testing)

Anyway - looks like this may be a "good enough" solution for out1 if
it can manage an ~2Gb file with 6Gb of memory available.  I'll test
that later today - but I should drag myself into the office now...

Bron.

(patch left attached below for reference)

> Only *after* testing this change is it probably a good idea to test the 
> real hack of then removing the highmem_dirtyable_memory() thing. 
> 
> Peter? Andrew?
> 
> 		Linus
> 
> ---
>  mm/page-writeback.c |    8 --------
>  1 files changed, 0 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 81a91e6..d55cfca 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -297,20 +297,12 @@ get_dirty_limits(long *pbackground, long *pdirty, long *pbdi_dirty,
>  {
>  	int background_ratio;		/* Percentages */
>  	int dirty_ratio;
> -	int unmapped_ratio;
>  	long background;
>  	long dirty;
>  	unsigned long available_memory = determine_dirtyable_memory();
>  	struct task_struct *tsk;
>  
> -	unmapped_ratio = 100 - ((global_page_state(NR_FILE_MAPPED) +
> -				global_page_state(NR_ANON_PAGES)) * 100) /
> -					available_memory;
> -
>  	dirty_ratio = vm_dirty_ratio;
> -	if (dirty_ratio > unmapped_ratio / 2)
> -		dirty_ratio = unmapped_ratio / 2;
> -
>  	if (dirty_ratio < 5)
>  		dirty_ratio = 5;
>