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
