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version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jun 24, 2025 at 10:13:49AM -0700, Alexei Starovoitov wrote: > On Thu, May 8, 2025 at 6:03 PM Harry Yoo wrote: > > > + s = kmalloc_slab(size, NULL, alloc_gfp, _RET_IP_); > > > + > > > + if (!(s->flags & __CMPXCHG_DOUBLE)) > > > + /* > > > + * kmalloc_nolock() is not supported on architectures that > > > + * don't implement cmpxchg16b. > > > + */ > > > + return NULL; > > > > Hmm when someone uses slab debugging flags (e.g., passing boot > > parameter slab_debug=FPZ as a hardening option on production [1], or > > just for debugging), __CMPXCHG_DOUBLE is not set even when the arch > > supports it. > > > > Is it okay to fail all kmalloc_nolock() calls in such cases? > > I studied the code and the git history. > Looks like slub doesn't have to disable cmpxchg mode when slab_debug is on. A slight correction; Debug caches do not use cmpxchg mode at all by design. If a future change enables cmpxchg mode for them, it will cause the same consistency issue. > The commit 41bec7c33f37 ("mm/slub: remove slab_lock() usage for debug > operations") > removed slab_lock from debug validation checks. An excellent point! Yes, SLUB does not maintain cpu slab and percpu partial slabs on debug caches. Alloc/free is done under n->list_lock, so no need for cmpxchg double and slab_lock() at all :) > So right now slab_lock() only serializes slab->freelist/counter update. > It's still necessary on arch-s that don't have cmpxchg, but that's it. > Only __update_freelist_slow() is using it. Yes. > The comment next to SLAB_NO_CMPXCHG is obsolete as well. > It's been there since days that slab_lock() was taken during > consistency checks. Yes. > I think the following diff is appropriate: > > diff --git a/mm/slub.c b/mm/slub.c > index 044e43ee3373..9d615cfd1b6f 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -286,14 +286,6 @@ static inline bool > kmem_cache_has_cpu_partial(struct kmem_cache *s) > #define DEBUG_DEFAULT_FLAGS (SLAB_CONSISTENCY_CHECKS | SLAB_RED_ZONE | \ > SLAB_POISON | SLAB_STORE_USER) > > -/* > - * These debug flags cannot use CMPXCHG because there might be consistency > - * issues when checking or reading debug information > - */ > -#define SLAB_NO_CMPXCHG (SLAB_CONSISTENCY_CHECKS | SLAB_STORE_USER | \ > - SLAB_TRACE) > - > - > > /* > * Debugging flags that require metadata to be stored in the slab. These get > * disabled when slab_debug=O is used and a cache's min order increases with > @@ -6654,7 +6646,7 @@ int do_kmem_cache_create(struct kmem_cache *s, > const char *name, > } > > #ifdef system_has_freelist_aba > - if (system_has_freelist_aba() && !(s->flags & SLAB_NO_CMPXCHG)) { > + if (system_has_freelist_aba()) { > /* Enable fast mode */ > s->flags |= __CMPXCHG_DOUBLE; > } > > It survived my stress tests. > Thoughts? Perhaps it's better to change the condition in kmalloc_nolock_noprof() from; if (!(s->flags & __CMPXCHG_DOUBLE)) return NULL; to; if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s)) return NULL; Because debug caches do not use cmpxchg double (and that's why it survived your test), it is not accurate to set __CMPXCHG_DOUBLE. And it'll get into trouble anyway if debug caches use cmpxchg double. -- Cheers, Harry / Hyeonggon