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

[Michal Hocko]

On Mon 17-12-18 04:25:46, Matthew Wilcox wrote:
> 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.

We could still panic the system on pre-mature OOM. So it doesn't really
seem good.

> > @@ -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.

Hmm, this is more interesting then. If there is any memcg accounted
allocation down that path _and_ the squashfs writeout can lock more
pages and mark them writeback before they are really sent to the storage
then we have a problem. See [1]

[1] http://lkml.kernel.org/r/20181213092221.27270-1-mhocko@xxxxxxxxxx

-- 
Michal Hocko
SUSE Labs