Re: [PATCH 01/13] mm/mmap: free gathered pages before MMF_OOM_SKIP

[Gregory Price (Meta)]

On 2026-11-11 18:13 +0000, Lorenzo Stoakes (ARM) wrote:
> The OOM killer can quickly trigger again after having selected a
> victim. That victim might not have freed its memory yet, but its mm will
> be marked MMF_OOM_SKIP by this point, so the OOM killer will ignore it.
> 
> This can cause the OOM killer to kill an innocent process when the first
> might have freed enough memory to relieve the memory pressure.
> 
> Fix the issue by freeing the memory before marking the mm MMF_OOM_SKIP.
> 
> Note that the task which calls mmput() might not be the victim - if it
> took a reference on the mm (e.g. ptrace), it may hold the last reference
> and release it when it calls mmput().
> 
> Therefore we cannot rely on current to tell whether the mm belongs to an
> OOM victim. Instead, partially revert commit b3541d912a84 ("mm: delete
> unused MMF_OOM_VICTIM flag") to reinstate MMF_OOM_VICTIM, which
> mark_oom_victim() sets on the victim's mm, and check that.
> 
> Reported-by: Chris Mason <mason@xxxxxxxxxx>
> Fixes: bf3980c85212 ("mm: drop oom code from exit_mmap")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

I was able to reproduce/observe the unfreed batch and the second victim
issue.  In the base series i could kill a second victim with the
in-flight oom in about 26/512 attempts, and 0/512 attempts with this
patch.

Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
Tested-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>

-- 
~Gregory