Re: System freezes after OOM

[Michal Hocko]

[CC David]

On Wed 13-07-16 22:19:23, Tetsuo Handa wrote:
> >> On Mon 11-07-16 11:43:02, Mikulas Patocka wrote:
> >> [...]
> >>> The general problem is that the memory allocator does 16 retries to 
> >>> allocate a page and then triggers the OOM killer (and it doesn't take into 
> >>> account how much swap space is free or how many dirty pages were really 
> >>> swapped out while it waited).
> >>
> >> Well, that is not how it works exactly. We retry as long as there is a
> >> reclaim progress (at least one page freed) back off only if the
> >> reclaimable memory can exceed watermks which is scaled down in 16
> >> retries. The overal size of free swap is not really that important if we
> >> cannot swap out like here due to complete memory reserves depletion:
> >> https://okozina.fedorapeople.org/bugs/swap_on_dmcrypt/vmlog-1462458369-00000/sample-00011/dmesg:
> >> [   90.491276] Node 0 DMA free:0kB min:60kB low:72kB high:84kB active_anon:4096kB inactive_anon:4636kB active_file:212kB inactive_file:280kB unevictable:488kB isolated(anon):0kB isolated(file):0kB present:15992kB managed:15908kB mlocked:488kB dirty:276kB writeback:4636kB mapped:476kB shmem:12kB slab_reclaimable:204kB slab_unreclaimable:4700kB kernel_stack:48kB pagetables:120kB unstable:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB writeback_tmp:0kB pages_scanned:61132 all_unreclaimable? yes
> >> [   90.491283] lowmem_reserve[]: 0 977 977 977
> >> [   90.491286] Node 0 DMA32 free:0kB min:3828kB low:4824kB high:5820kB active_anon:423820kB inactive_anon:424916kB active_file:17996kB inactive_file:21800kB unevictable:20724kB isolated(anon):384kB isolated(file):0kB present:1032184kB managed:1001260kB mlocked:20724kB dirty:25236kB writeback:49972kB mapped:23076kB shmem:1364kB slab_reclaimable:13796kB slab_unreclaimable:43008kB kernel_stack:2816kB pagetables:7320kB unstable:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB writeback_tmp:0kB pages_scanned:5635400 all_unreclaimable? yes
> >>
> >> Look at the amount of free memory. It is completely depleted. So it
> >> smells like a process which has access to memory reserves has consumed
> >> all of it. I suspect a __GFP_MEMALLOC resp. PF_MEMALLOC from softirq
> >> context user which went off the leash.
> > 
> > It is caused by the commit f9054c70d28bc214b2857cf8db8269f4f45a5e23. Prior 
> > to this commit, mempool allocations set __GFP_NOMEMALLOC, so they never 
> > exhausted reserved memory. With this commit, mempool allocations drop 
> > __GFP_NOMEMALLOC, so they can dig deeper (if the process has PF_MEMALLOC, 
> > they can bypass all limits).
> 
> I wonder whether commit f9054c70d28bc214 ("mm, mempool: only set
> __GFP_NOMEMALLOC if there are free elements") is doing correct thing.
> It says
> 
>     If an oom killed thread calls mempool_alloc(), it is possible that it'll
>     loop forever if there are no elements on the freelist since
>     __GFP_NOMEMALLOC prevents it from accessing needed memory reserves in
>     oom conditions.

I haven't studied the patch very deeply so I might be missing something
but from a quick look the patch does exactly what the above says.

mempool_alloc used to inhibit ALLOC_NO_WATERMARKS by default. David has
only changed that to allow ALLOC_NO_WATERMARKS if there are no objects
in the pool and so we have no fallback for the default __GFP_NORETRY
request.

> but we can allow mempool_alloc(__GFP_NOMEMALLOC) requests to access
> memory reserves via below change, can't we?

Well, I do not see all the potential side effects of such a change but
I believe it shouldn't be really necessary because we should eventually
allow ALLOC_NO_WATERMARKS even from mempool_alloc.

> The purpose of allowing
> ALLOC_NO_WATERMARKS via TIF_MEMDIE is to make sure current allocation
> request does not to loop forever inside the page allocator, isn't it?
> Why we need to allow mempool_alloc(__GFP_NOMEMALLOC) requests to use
> ALLOC_NO_WATERMARKS when TIF_MEMDIE is not set?
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6903b69..e4e3700 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3439,14 +3439,14 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  	} else if (unlikely(rt_task(current)) && !in_interrupt())
>  		alloc_flags |= ALLOC_HARDER;
>  
> -	if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
> +	if (!in_interrupt() && unlikely(test_thread_flag(TIF_MEMDIE)))
> +		alloc_flags |= ALLOC_NO_WATERMARKS;
> +	else if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
>  		if (gfp_mask & __GFP_MEMALLOC)
>  			alloc_flags |= ALLOC_NO_WATERMARKS;
>  		else if (in_serving_softirq() && (current->flags & PF_MEMALLOC))
>  			alloc_flags |= ALLOC_NO_WATERMARKS;
> -		else if (!in_interrupt() &&
> -				((current->flags & PF_MEMALLOC) ||
> -				 unlikely(test_thread_flag(TIF_MEMDIE))))
> +		else if (!in_interrupt() && (current->flags & PF_MEMALLOC))
>  			alloc_flags |= ALLOC_NO_WATERMARKS;
>  	}
>  #ifdef CONFIG_CMA
> 
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-- 
Michal Hocko
SUSE Labs