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

[Matthew Wilcox <willy@infradead.org>]

On Mon, Dec 17, 2018 at 07:51:27PM +0900, Tetsuo Handa wrote:
> 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

Too far; I meant one line earlier, before we try to select a different
process.

> @@ -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" ?

It's worth noticing that squashfs _is_ in fact holding a page locked in
squashfs_copy_cache() when it calls grab_cache_page_nowait().  I'm not
sure if this will lead to trouble or not because I'm insufficiently
familiar with the reclaim path.