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

[Tejun Heo <tj@kernel.org>]

Hello,

On Wed, Feb 12, 2025 at 05:57:04PM +0100, Michal Hocko wrote:
...
> 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

I see. Nice.

> 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?

Longer term, yeah, I think so.

> > 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.

Hmm... you'd a better judge on whether that'd be okay or not but it does
bother me that we might be increasing the chance of allocation failures for
GFP_KERNEL users at least under memory pressure.

> > 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.

Right, the current locking in expansion path is really simple because it was
assuming everyone would be doing GFP_KERNEL allocation. We'd have to break
up the locking so that allocations are done outside locking, which hopefully
shouldn't be too complicated.

Thanks.

-- 
tejun