Re: [PATCH] squashfs: enable __GFP_FS in ->readpage to prevent hang in mem alloc

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

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" ?