Re: [patch 00/12] mm: page_alloc: improve OOM mechanism and policy

[Michal Hocko]

[Sorry for a late reply]

On Tue 07-04-15 10:18:22, Johannes Weiner wrote:
> On Wed, Apr 01, 2015 at 05:19:20PM +0200, Michal Hocko wrote:
> > On Mon 30-03-15 11:32:40, Dave Chinner wrote:
> > > On Fri, Mar 27, 2015 at 11:05:09AM -0400, Johannes Weiner wrote:
> > [...]
> > > > GFP_NOFS sites are currently one of the sites that can deadlock inside
> > > > the allocator, even though many of them seem to have fallback code.
> > > > My reasoning here is that if you *have* an exit strategy for failing
> > > > allocations that is smarter than hanging, we should probably use that.
> > > 
> > > We already do that for allocations where we can handle failure in
> > > GFP_NOFS conditions. It is, however, somewhat useless if we can't
> > > tell the allocator to try really hard if we've already had a failure
> > > and we are already in memory reclaim conditions (e.g. a shrinker
> > > trying to clean dirty objects so they can be reclaimed).
> > > 
> > > From that perspective, I think that this patch set aims force us
> > > away from handling fallbacks ourselves because a) it makes GFP_NOFS
> > > more likely to fail, and b) provides no mechanism to "try harder"
> > > when we really need the allocation to succeed.
> > 
> > You can ask for this "try harder" by __GFP_HIGH flag. Would that help
> > in your fallback case?
> 
> I would think __GFP_REPEAT would be more suitable here.  From the doc:
> 
>  * __GFP_REPEAT: Try hard to allocate the memory, but the allocation attempt
>  * _might_ fail.  This depends upon the particular VM implementation.
> 
> so we can make the semantics of GFP_NOFS | __GFP_REPEAT such that they
> are allowed to use the OOM killer and dip into the OOM reserves.

__GFP_REPEAT is quite subtle already. It makes a difference only for
high order allocations and it is not clear to me why it should imply OOM
killer for small orders now. Or did you suggest making it special only
with GFP_NOFS? That sounds even more ugly.

AFAIU, David wasn't asking for the OOM killer as much as he was
interested in getting access to a small amount of reserves in order to
make a progress. __GFP_HIGH is there for this purpose.

> My question here would be: are there any NOFS allocations that *don't*
> want this behavior?  Does it even make sense to require this separate
> annotation or should we just make it the default?
> 
> The argument here was always that NOFS allocations are very limited in
> their reclaim powers and will trigger OOM prematurely.  However, the
> way we limit dirty memory these days forces most cache to be clean at
> all times, and direct reclaim in general hasn't been allowed to issue
> page writeback for quite some time.  So these days, NOFS reclaim isn't
> really weaker than regular direct reclaim. 

What about [di]cache and some others fs specific shrinkers (and heavy
metadata loads)?

> The only exception is that
> it might block writeback, so we'd go OOM if the only reclaimables left
> were dirty pages against that filesystem.  That should be acceptable.

OOM killer is hardly acceptable by most users I've heard from. OOM
killer is the _last_ resort and if the allocation is restricted then
we shouldn't use the big hammer. The allocator might use __GFP_HIGH to
get access to memory reserves if it can fail or __GFP_NOFAIL if it
cannot. With your patches the NOFAIL case would get an access to memory
reserves as well. So I do not really see a reason to change GFP_NOFS vs.
OOM killer semantic.

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 47981c5e54c3..fe3cb2b0b85b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2367,16 +2367,6 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>  		/* The OOM killer does not needlessly kill tasks for lowmem */
>  		if (ac->high_zoneidx < ZONE_NORMAL)
>  			goto out;
> -		/* The OOM killer does not compensate for IO-less reclaim */
> -		if (!(gfp_mask & __GFP_FS)) {
> -			/*
> -			 * XXX: Page reclaim didn't yield anything,
> -			 * and the OOM killer can't be invoked, but
> -			 * keep looping as per tradition.
> -			 */
> -			*did_some_progress = 1;
> -			goto out;
> -		}
>  		if (pm_suspended_storage())
>  			goto out;
>  		/* The OOM killer may not free memory on a specific node */

-- 
Michal Hocko
SUSE Labs
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