Re: [PATCH] mm, percpu: do not consider sleepable allocations atomic

[Michal Hocko <mhocko@suse.com>]

On Tue 11-02-25 10:55:20, Tejun Heo wrote:
> Hello, Michal.
> 
> On Thu, Feb 06, 2025 at 01:26:33PM +0100, Michal Hocko wrote:
> ...
> > It has turned out that iscsid has worked around this by dropping
> > PR_SET_IO_FLUSHER (https://github.com/open-iscsi/open-iscsi/pull/382)
> > when scanning host. But we can do better in this case on the kernel side
> 
> FWIW, requiring GFP_KERNEL context for probing doesn't sound too crazy to
> me.
> 
> > @@ -2204,7 +2204,12 @@ static void pcpu_balance_workfn(struct work_struct *work)
> >  	 * to grow other chunks.  This then gives pcpu_reclaim_populated() time
> >  	 * to move fully free chunks to the active list to be freed if
> >  	 * appropriate.
> > +	 *
> > +	 * Enforce GFP_NOIO allocations because we have pcpu_alloc users
> > +	 * constrained to GFP_NOIO/NOFS contexts and they could form lock
> > +	 * dependency through pcpu_alloc_mutex
> >  	 */
> > +	unsigned int flags = memalloc_noio_save();
> 
> Just for context, the reason why the allocation mask support was limited to
> GFP_KERNEL or not rather than supporting full range of GFP flags is because
> percpu memory area expansion can involve page table allocations in the
> vmalloc area which always uses GFP_KERNEL. memalloc_noio_save() masks IO
> part out of that, right? It might be worthwhile to explain why we aren't
> passing down GPF flags throughout and instead depending on masking.

I have gone with masking because that seemed easier to review and more
robust solution. vmalloc does support NOFS/NOIO contexts these days (it
will just uses scoped masking in those cases). Propagating the gfp
throughout the worker code path is likely possible, but I haven't really
explored that in detail to be sure. Would that be preferable even if the
fix would be more involved?

> Also, doesn't the above always prevent percpu allocations from doing fs/io
> reclaims? 

Yes it does. Probably worth mentioning in the changelog. These
allocations should be rare so having a constrained reclaim didn't really
seem problematic to me. There should be kswapd running in the background
with the full reclaim power.

> ie. Shouldn't the masking only be used if the passed in gfp
> doesn't allow fs/io?

This is a good question. I have to admit that my understanding might be
incorrect but wouldn't it be possible that we could get the lock
dependency chain if GFP_KERNEL and scoped NOFS alloc_pcp calls are
competing? 

					fs/io lock
					pcpu_alloc_noprof(NOFS/NOIO)
pcpu_alloc_noprof(GFP_KERNEL)
  pcpu_schedule_balance_work
    pcpu_alloc_mutex
    					  pcpu_alloc_mutex
      allocation_deadlock throgh fs/io lock

This is currently not possible because constrained allocations only do
trylock.

Makes sense?
-- 
Michal Hocko
SUSE Labs