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From: Hao Li To: "Vlastimil Babka (SUSE)" Cc: Harry Yoo , Christoph Lameter , David Rientjes , Roman Gushchin , Suren Baghdasaryan , Alexei Starovoitov , Andrew Morton , Johannes Weiner , Michal Hocko , Shakeel Butt , Alexander Potapenko , Marco Elver , Dmitry Vyukov , kasan-dev@googlegroups.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org Subject: Re: [PATCH v2 15/16] mm/slab: remove __GFP_NO_OBJ_EXT usage from alloc_slab_obj_exts() Message-ID: References: <20260610-slab_alloc_flags-v2-0-7190909db118@kernel.org> <20260610-slab_alloc_flags-v2-15-7190909db118@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260610-slab_alloc_flags-v2-15-7190909db118@kernel.org> X-Migadu-Flow: FLOW_OUT On Wed, Jun 10, 2026 at 05:40:17PM +0200, Vlastimil Babka (SUSE) wrote: > __GFP_NO_OBJ_EXT has limited scope within the slab allocator itself and > gfp flags are a scarce resource, unlike slab's alloc_flags. > > Introduce SLAB_ALLOC_NO_RECURSE alloc flag that has the same intent as > __GFP_NO_OBJ_EXT but a more generic name, meaning that a kmalloc() > family function should not recurse into another kmalloc*() for the > purposes of allocating auxiliary structures (obj_ext arrays or sheaves). > > First, replace the __GFP_NO_OBJ_EXT for allocating obj_ext arrays in > alloc_slab_obj_exts(). Make use of the newly added kmalloc_flags() > function, where we can pass alloc_flags with SLAB_ALLOC_NO_RECURSE > added. This will also pass through SLAB_ALLOC_TRYLOCK so we don't need > to special case kmalloc_nolock() anymore. > > Note that until now the kmalloc_nolock() ignored the incoming gfp flags > and hardcoded __GFP_ZERO | __GFP_NO_OBJ_EXT. But it's correct to pass on > the incoming gfp flags (only augmented with __GFP_ZERO), because if > alloc_flags contain SLAB_ALLOC_TRYLOCK, the incoming gfp flags have to > be also compatible with it. > > Signed-off-by: Vlastimil Babka (SUSE) > --- > mm/slab.h | 1 + > mm/slub.c | 13 +++++-------- > 2 files changed, 6 insertions(+), 8 deletions(-) > > diff --git a/mm/slab.h b/mm/slab.h > index 45bfcfb35a9c..509f330654b8 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -21,6 +21,7 @@ > #define SLAB_ALLOC_DEFAULT 0x00 /* no flags */ > #define SLAB_ALLOC_TRYLOCK 0x01 /* a kmalloc_nolock() allocation */ > #define SLAB_ALLOC_NEW_SLAB 0x02 /* a flag for alloc_slab_obj_exts() */ > +#define SLAB_ALLOC_NO_RECURSE 0x04 /* prevent kmalloc() recursion */ > > static inline bool alloc_flags_allow_spinning(const unsigned int alloc_flags) > { > diff --git a/mm/slub.c b/mm/slub.c > index cbb38bd01e46..7dfbd0251aa2 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2167,15 +2167,12 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > > gfp &= ~OBJCGS_CLEAR_MASK; > /* Prevent recursive extension vector allocation */ > - gfp |= __GFP_NO_OBJ_EXT; > + alloc_flags |= SLAB_ALLOC_NO_RECURSE; > > sz = obj_exts_alloc_size(s, slab, gfp); > For the original calls to kmalloc_nolock and kmalloc_node, I notice a difference: > - if (unlikely(!allow_spin)) > - vec = kmalloc_nolock(sz, __GFP_ZERO | __GFP_NO_OBJ_EXT, > - slab_nid(slab)); kmalloc_nolock completely discarded `gfp` flags. > - else > - vec = kmalloc_node(sz, gfp | __GFP_ZERO, slab_nid(slab)); while kmalloc_node preserved and passed it along. > + /* This will use kmalloc_nolock() if alloc_flags say so */ > + vec = kmalloc_flags(sz, gfp | __GFP_ZERO, alloc_flags, slab_nid(slab)); Now both paths are merged into kmalloc_flags, the gfp flags are unconditionally carried through. It seems this might carry some unwanted flags. I traced the call path and found that ___slab_alloc sets the __GFP_THISNODE for trynode_flags. If this flag propagates all the way into kmalloc_flags->...->__kmalloc_nolock_noprof, it will trigger the VM_WARN_ON_ONCE warning. Maybe we need to strip the original gfp if `!allow_spin`. -- Thanks, Hao