Re: [PATCH] mm: page_alloc: unreserve highatomic page blocks before oom

[Pavan Kondeti <quic_pkondeti@quicinc.com>]

On Mon, Oct 30, 2023 at 06:09:50PM +0530, Charan Teja Kalla wrote:
> __alloc_pages_direct_reclaim() is called from slowpath allocation where
> high atomic reserves can be unreserved after there is a progress in
> reclaim and yet no suitable page is found. Later should_reclaim_retry()
> gets called from slow path allocation to decide if the reclaim needs to
> be retried before OOM kill path is taken.
> 
> should_reclaim_retry() checks the available(reclaimable + free pages)
> memory against the min wmark levels of a zone and returns:
> a)  true, if it is above the min wmark so that slow path allocation will
> do the reclaim retries.
> b) false, thus slowpath allocation takes oom kill path.
> 
> should_reclaim_retry() can also unreserves the high atomic reserves
> **but only after all the reclaim retries are exhausted.**
> 
> In a case where there are almost none reclaimable memory and free pages
> contains mostly the high atomic reserves but allocation context can't
> use these high atomic reserves, makes the available memory below min
> wmark levels hence false is returned from should_reclaim_retry() leading
> the allocation request to take OOM kill path. This is an early oom kill
> because high atomic reserves are holding lot of free memory and 
> unreserving of them is not attempted.
> 
> (early)OOM is encountered on a machine in the below state(excerpt from
> the oom kill logs):
> [  295.998653] Normal free:7728kB boost:0kB min:804kB low:1004kB
> high:1204kB reserved_highatomic:8192KB active_anon:4kB inactive_anon:0kB
> active_file:24kB inactive_file:24kB unevictable:1220kB writepending:0kB
> present:70732kB managed:49224kB mlocked:0kB bounce:0kB free_pcp:688kB
> local_pcp:492kB free_cma:0kB
> [  295.998656] lowmem_reserve[]: 0 32
> [  295.998659] Normal: 508*4kB (UMEH) 241*8kB (UMEH) 143*16kB (UMEH)
> 33*32kB (UH) 7*64kB (UH) 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB
> 0*4096kB = 7752kB
> 
> Per above log, the free memory of ~7MB exist in the high atomic
> reserves is not freed up before falling back to oom kill path.
> 
> This fix includes unreserving these atomic reserves in the OOM path
> before going for a kill. The side effect of unreserving in oom kill path
> is that these free pages are checked against the high wmark. If
> unreserved from should_reclaim_retry()/__alloc_pages_direct_reclaim(),
> they are checked against the min wmark levels.
> 
> Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>

Thanks for the detailed commit description. Really helpful in
understanding the problem you are fixing.

> ---
>  mm/page_alloc.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 95546f3..2a2536d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3281,6 +3281,8 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
>  		.order = order,
>  	};
>  	struct page *page;
> +	struct zone *zone;
> +	struct zoneref *z;
>  
>  	*did_some_progress = 0;
>  
> @@ -3295,6 +3297,16 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
>  	}
>  
>  	/*
> +	 * If should_reclaim_retry() encounters a state where:
> +	 * reclaimable + free doesn't satisfy the wmark levels,
> +	 * it can directly jump to OOM without even unreserving
> +	 * the highatomic page blocks. Try them for once here
> +	 * before jumping to OOM.
> +	 */
> +retry:
> +	unreserve_highatomic_pageblock(ac, true);
> +

Not possible to fix this in should_reclaim_retry()? 

> +	/*
>  	 * Go through the zonelist yet one more time, keep very high watermark
>  	 * here, this is only to catch a parallel oom killing, we must fail if
>  	 * we're still under heavy pressure. But make sure that this reclaim
> @@ -3307,6 +3319,12 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
>  	if (page)
>  		goto out;
>  
> +	for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->highest_zoneidx,
> +								ac->nodemask) {
> +		if (zone->nr_reserved_highatomic > 0)
> +			goto retry;
> +	}
> +
>  	/* Coredumps can quickly deplete all memory reserves */
>  	if (current->flags & PF_DUMPCORE)
>  		goto out;