Re: [PATCH] mm, oom: allow oom reaper to race with exit_mmap

[Michal Hocko]

On Tue 25-07-17 18:31:10, Kirill A. Shutemov wrote:
> On Tue, Jul 25, 2017 at 05:23:00PM +0200, Michal Hocko wrote:
> > what is stdev?
> 
> Updated tables:
> 
> 3 runs before the patch:
>    Min. 1st Qu.  Median    Mean 3rd Qu.    Max.  Stdev
>  177200  205000  212900  217800  223700 2377000  32868
>  172400  201700  209700  214300  220600 1343000  31191
>  175700  203800  212300  217100  223000 1061000  31195
> 
> 3 runs after the patch:
>    Min. 1st Qu.  Median    Mean 3rd Qu.    Max.  Stdev
>  175900  204800  213000  216400  223600 1989000  27210
>  180300  210900  219600  223600  230200 3184000  32609
>  182100  212500  222000  226200  232700 1473000  32138

High std/avg ~15% matches my measurements (mine were even higher ~20%)
and that would suggest that 3% average difference is still somehing
within a "noise".

Anyway, I do not really need to take the lock unless the task is the
oom victim. Could you try whether those numbers improve if the lock is
conditional?

Thanks!
---
diff --git a/mm/mmap.c b/mm/mmap.c
index 0eeb658caa30..ca8a274485f8 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -44,6 +44,7 @@
 #include <linux/userfaultfd_k.h>
 #include <linux/moduleparam.h>
 #include <linux/pkeys.h>
+#include <linux/oom.h>
 
 #include <linux/uaccess.h>
 #include <asm/cacheflush.h>
@@ -2997,7 +2998,8 @@ void exit_mmap(struct mm_struct *mm)
 	 * oom reaper might race with exit_mmap so make sure we won't free
 	 * page tables or unmap VMAs under its feet
 	 */
-	down_write(&mm->mmap_sem);
+	if (tsk_is_oom_victim(current))
+		down_write(&mm->mmap_sem);
 	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
 	tlb_finish_mmu(&tlb, 0, -1);
 
@@ -3012,7 +3014,8 @@ void exit_mmap(struct mm_struct *mm)
 	}
 	mm->mmap = NULL;
 	vm_unacct_memory(nr_accounted);
-	up_write(&mm->mmap_sem);
+	if (tsk_is_oom_victim(current))
+		up_write(&mm->mmap_sem);
 }
 
 /* Insert vm structure into process list sorted by address
-- 
Michal Hocko
SUSE Labs