Re: [RESEND PATCH V2] mm: page_alloc: unreserve highatomic page blocks before oom

[David Rientjes]

On Fri, 24 Nov 2023, 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 can turn into a early
> oom kill if high atomic reserves are holding lot of free memory and
> unreserving of them is not attempted.
> 
> (early)OOM is encountered on a VM with the below state:
> [  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.
> 
> Fix it by trying to unreserve the high atomic reserves in
> should_reclaim_retry() before __alloc_pages_direct_reclaim() can
> fallback to oom kill path.
> 
> Fixes: 0aaa29a56e4f ("mm, page_alloc: reserve pageblocks for high-order atomic allocations on demand")
> Reported-by: Chris Goldsworthy <quic_cgoldswo@xxxxxxxxxxx>
> Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>