Re: [PATCH v2 05/16] mm/slab: introduce alloc_flags and SLAB_ALLOC_TRYLOCK

[Harry Yoo]

On 6/11/26 12:40 AM, Vlastimil Babka (SUSE) wrote:
> Similarly to the page allocators, introduce slab-allocator specific
> alloc flags that internally control allocation behavior in addition to
> gfp_flags, without occupying the limited gfp flags space.
> 
> Introduce the first flag SLAB_ALLOC_TRYLOCK that behaves similarly to
> page allocator's ALLOC_TRYLOCK and will be used to reimplement
> kmalloc_nolock()'s "!allow_spin" behavior. That currently relies on
> gfpflags_allow_spinning() and thus the lack of both __GFP_RECLAIM flags,
> importantly __GFP_KSWAPD_RECLAIM. This can give false-positive results
> e.g. in early boot with a restricted gfp_allowed_mask.
> 
> Also introduce alloc_flags_allow_spinning() to replace the usage of
> gfpflags_allow_spinning().
> 
> Start using alloc_flags and the new check first in alloc_from_pcs() and
> __pcs_replace_empty_main(). This means some slab allocations that were
> falsely treated as kmalloc_nolock() due to their gfp flags will now have
> higher chances of succeed, and this will further increase with followup
> changes.
> 
> Remove a WARN_ON_ONCE() from refill_objects() as it's now legitimate to
> reach it from a slab allocation that's not _nolock() and yet lacks
> __GFP_KSWAPD_RECLAIM for other reasons.
> 
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
> ---
>  mm/slab.h |  9 +++++++++
>  mm/slub.c | 17 ++++++++---------
>  2 files changed, 17 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 1bf9c3021ae3..96f65b625600 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -4664,7 +4665,7 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
>  		return NULL;
>  	}
>  
> -	allow_spin = gfpflags_allow_spinning(gfp);
> +	allow_spin = alloc_flags_allow_spinning(alloc_flags);

Sashiko wrote [1]:
> Does this bypass the caller's gfp constraints for standard allocations?
> Looking at slab_alloc_node(), standard allocations now pass
> SLAB_ALLOC_DEFAULT into alloc_from_pcs():
> -	object = alloc_from_pcs(s, gfpflags, node);
> +	object = alloc_from_pcs(s, gfpflags, SLAB_ALLOC_DEFAULT, node);
> This default flag means alloc_flags_allow_spinning() will unconditionally
> return true regardless of the gfp flags provided.

Yes, but that's not used in _nolock path
as mentioned in patch 6 description :)

> If a caller allocating under a raw spinlock intentionally strips
> __GFP_KSWAPD_RECLAIM (for example, by using __GFP_NOWARN) to prevent
> sleeping,

That's a horrible hack (and hypothetical. Nobody should be stripping
__GFP_KSWAP_RECLAIM instead of using kmalloc_nolock(). That's purely
broken).

> won't this allow the allocator to execute spin_lock_irqsave()
> on barn->lock or n->list_lock?
>
> On systems with preempt-rt enabled, a standard spinlock maps to a sleeping
> lock, so taking these locks in an atomic context could cause a scheduling
> while atomic panic.
>
> Since there is no nolock variant available for custom caches, do callers
> currently have any alternative mitigation?

Well, RT kernels are not supposed to allocate meomry under a raw
spinlock (at least w/ allow_spin = true)

[1]
https://sashiko.dev/#/patchset/20260610-slab_alloc_flags-v2-0-7190909db118%40kernel.org

-- 
Cheers,
Harry / Hyeonggon

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