On 2018/12/17 18:33, Michal Hocko wrote:
> On Sun 16-12-18 19:51:57, Matthew Wilcox wrote:
> [...]
>> Ah, yes, that makes perfect sense. Thank you for the explanation.
>>
>> I wonder if the correct fix, however, is not to move the check for
>> GFP_NOFS in out_of_memory() down to below the check whether to kill
>> the current task. That would solve your problem, and I don't _think_
>> it would cause any new ones. Michal, you touched this code last, what
>> do you think?
>
> What do you mean exactly? Whether we kill a current task or something
> else doesn't change much on the fact that NOFS is a reclaim restricted
> context and we might kill too early. If the fs can do GFP_FS then it is
> obviously a better thing to do because FS metadata can be reclaimed as
> well and therefore there is potentially less memory pressure on
> application data.
>
I interpreted "to move the check for GFP_NOFS in out_of_memory() down to
below the check whether to kill the current task" as
@@ -1077,15 +1077,6 @@ bool out_of_memory(struct oom_control *oc)
}
/*
- * 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.
- */
- if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS))
- return true;
-
- /*
* Check if there were limitations on the allocation (only relevant for
* NUMA and memcg) that may require different handling.
*/
@@ -1104,6 +1095,19 @@ bool out_of_memory(struct oom_control *oc)
}
select_bad_process(oc);
+
+ /*
+ * 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.
+ */
+ if ((oc->gfp_mask && !(oc->gfp_mask & __GFP_FS)) && oc->chosen &&
+ oc->chosen != (void *)-1UL && oc->chosen != current) {
+ put_task_struct(oc->chosen);
+ return true;
+ }
+
/* Found nothing?!?! */
if (!oc->chosen) {
dump_header(oc, NULL);
which is prefixed by "the correct fix is not".
Behaving like sysctl_oom_kill_allocating_task == 1 if __GFP_FS is not used
will not be the correct fix. But ...
Hou Tao wrote:
> There is no need to disable __GFP_FS in ->readpage:
> * It's a read-only fs, so there will be no dirty/writeback page and
> there will be no deadlock against the caller's locked page
is read-only filesystem sufficient for safe to use __GFP_FS?
Isn't "whether it is safe to use __GFP_FS" depends on "whether fs locks
are held or not" rather than "whether fs has dirty/writeback page or not" ?
