Re: [PATCH 1/1] mm/oom_kill: trigger the oom killer if oom occurs without __GFP_FS

[Michal Hocko <mhocko@suse.com>]

On Thu 27-04-23 11:47:10, Hui Wang wrote:
[...]
> So Michal,
> 
> Don't know if you read the "[PATCH 0/1] mm/oom_kill: system enters a state
> something like hang when running stress-ng", do you know why out_of_memory()
> will return immediately if there is no __GFP_FS, could we drop these lines
> directly:
> 
>     /*
>      * The OOM killer does not compensate for IO-less reclaim.
>      * pagefault_out_of_memory lost its gfp context so we have to
>      * make sure exclude 0 mask - all other users should have at least
>      * ___GFP_DIRECT_RECLAIM to get here. But mem_cgroup_oom() has to
>      * invoke the OOM killer even if it is a GFP_NOFS allocation.
>      */
>     if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS) && !is_memcg_oom(oc))
>         return true;

The comment is rather hard to grasp without an intimate knowledge of the
memory reclaim. The primary reason is that the allocation context
without __GFP_FS (and also __GFP_IO) cannot perform a full memory
reclaim because fs or the storage subsystem might be holding locks
required for the memory reclaim. This means that a large amount of
reclaimable memory is out of sight of the specific direct reclaim
context. If we allowed oom killer to trigger we could invoke the oom
killer while there is a lot of otherwise reclaimable memory. As you can
imagine not something many users would appreciate as the oom kill is a
very disruptive operation. In this case we rely on kswapd or other
GFP_KERNEL like allocation context to make forward instead. If there is
really nothing reclaimable then the oom killer would eventually hit from
elsewhere.

HTH
-- 
Michal Hocko
SUSE Labs