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b=dJvU2YxLmYXK/jl2u71imBIY4cLgK8q4YeB6PZOa48MXe9mzkP57bFHOVIwKbf5Ok Tc0CklWbTCnsIWPXoiP5U7rSWvUhf6TOfrN5ioOLPCTfjFfILsUUKaPQiBRapJJuG+ Q2uXLFPxjR5C3nmyZ/+NI4DqtT91KKd4YYRRnlUSmu6Nqp0wWUNmYF0IoY1vvYyiL/ OsJe8e35TU1xZGvitLtGggiEWNkyVQ9fKSVKC+yDIYxQkrdDjcwPQ6gJAGehnUCwF2 LHXy/9d0VGUGzmpKLYm0XU/m86qu+F9/TTHVpT+9dFHfSEIl3jJ2d3ORRxDcBmksCr FMEupNOtrQlFQ== Message-ID: Date: Mon, 15 Jun 2026 13:11:37 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 15/16] mm/slab: remove __GFP_NO_OBJ_EXT usage from alloc_slab_obj_exts() Content-Language: en-US To: Suren Baghdasaryan , Hao Li Cc: Harry Yoo , Christoph Lameter , David Rientjes , Roman Gushchin , 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, Hao Ge References: <20260610-slab_alloc_flags-v2-0-7190909db118@kernel.org> <20260610-slab_alloc_flags-v2-15-7190909db118@kernel.org> <49f1bf1e-fcaf-48fa-a7b1-f8ee78b19762@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: BB2D12000F X-Stat-Signature: nipj6j6gkr7cshr4dpua3zp4mthredkt X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1781521903-76427 X-HE-Meta: U2FsdGVkX1+yW6F5/0JUW3OM41Yl6Ufqxme3eF2qZvRs9MFcotX6IKIfOqjUcdEKJbIDkgf0juqCONDbRWN7dBYg/eFE0TvVB/VnnQugGSW3uCkB5qjQYANJqo1r2XSosMBgSLGMIwxKs74V/wYRkjmuRUDtZQzFKA1Eo97dXbJF7i3l9ehQMxHowA3rIBReBqk+N+7v9iUBwpP378f8X5pU8mwwzRwJoyH9cAKj9CGV2zPMWGgl3tru7pw/wOmaiz0yAX4eJAOA3pM8/r//hIkxpjxDHZKm3KKAMMeh+uf7554RT4YQ6fKg8V0OVllhJ36nrS9NYGZfvczkbpe1aQ4QdrcF9CumSt0o9p0KOAnsRQXYqZ6G+1/ydxoVcQAq5HM7cldSZi/NdwR2J/Uy4ZsBGL67pqeihEUSlHVAtVuhju74kBtbtw862+I9SDebV7Sp7KvWDM94WZQD2PkOupHsoGNsC9cIPYZ7CF5mtVDcZHst41dca70CR4NdWqPOPw18tyC0nRwk+X5TfICdYg5ebKzanSbvsMAx5hCZ+wCD+Kwl9d5nEZoIdOKduU78cVbU/Nz4rnpaRpryi1ITIb2VqPO/RqHv8376PDC/LD57/pKZTjBtjiRBWhEoFe0iuM0Q5a8cQssu1owYFzJOTxUuvnie3iJmhOySkOlrt5lG5m1yQ6fL+6Zl/CxcM9mP8zRh1WW0hTslpd1uyrVIgbiFW8zD06/V4NHKiis0JRpoLR6z05JloWhY1S0y0lP0z4CRPf8qk6UtRgd7YD6Y9Mf/0yVCSW+Es9bZZCJcv9ZfGqh7cUzhJpRqaaKsSTb/2uQV+/yyFdIDmD3VFCe39+scduIKL5Gf8QVaZSquLiUWyVZFkKFTesXmFVX0wEQgaUfUbi7YX9sTNCZ7jQtkIIGechk7dRsNkZJtU0NtocKNAVDbAsDIIaBoBoHegrv9i0gM+8OwOCBgMa8viOY ng2vNh3e scIMa9MCdqPxhlCmfDm9CwqIQdKb64NsUYIREdMi9qhw57p4du/9Ek5AIJHX3ERlC69KDrT5oZzfBlHJvud/V3vQlLMV1amLCCk1XrCT78E0xST+Ucp+LxRiNL+RaXm2bKvAXXFlwAl6woSnWxpDv9tPCR2fTJv1otLxPDaiNk5OtsE69eIpvkgn67Usx0HiGLMzIoXlyLHzIgRsHvhFcmsVK/YOgdktHFIA5KtaWsrYnXtWTjFSSQ/UwCiln7rr3jvQeeL+ZtnNXO9TvjDp497zyE49Xg8/2K9vSBI3D1f+amGC8oyJS0AHuBVytaJJZRmdVd+yhDTPF/D7QMiSQLmanSVyDOTfhdBYY Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/15/26 07:38, Suren Baghdasaryan wrote: > On Fri, Jun 12, 2026 at 4:30 AM Hao Li wrote: >> >> On Fri, Jun 12, 2026 at 12:17:45PM +0200, Vlastimil Babka (SUSE) wrote: >> > On 6/12/26 08:54, Hao Li wrote: >> > > 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. This should do the job in a more generic way I hope? >> > >> >> Yeah, this is more elegant. >> >> > diff --git a/mm/slub.c b/mm/slub.c >> > index f9b8dc56bb57..0bf53f70c9be 100644 >> > --- a/mm/slub.c >> > +++ b/mm/slub.c >> > @@ -2047,12 +2047,15 @@ static inline void dec_slabs_node(struct kmem_cache *s, int node, >> > #endif /* CONFIG_SLUB_DEBUG */ >> > >> > /* >> > - * The allocated objcg pointers array is not accounted directly. >> > + * The allocated objcg pointers array or sheaf is not accounted directly. >> > * Moreover, it should not come from DMA buffer and is not readily >> > - * reclaimable. So those GFP bits should be masked off. >> > + * reclaimable. Node restriction for the parent allocation also should >> > + * not apply to the slab's internal objects. >> > + * So those GFP bits should be masked off. >> > */ >> > #define OBJCGS_CLEAR_MASK (__GFP_DMA | __GFP_RECLAIMABLE | \ >> > - __GFP_ACCOUNT | __GFP_NOFAIL) >> > + __GFP_ACCOUNT | __GFP_NOFAIL | >> > + __GFP_THISNODE ) >> >> Good idea! Both code and comments make sense to me. > > Makes sense. I see > https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=slab/for-next > already implementing this and also keeping __GFP_NO_OBJ_EXT and > SLAB_ALLOC_NO_RECURSE both used. That version looks good to me, so > I'll wait for v3. OK. > At the end of this series, we end up with no users of __GFP_NO_OBJ_EXT > but we still keep it defined. I'm guessing you leave it because of the > new patch [1] which aliases __GFP_NO_OBJ_EXT? I will have to make that Yeah. > mechanism work without a GFP flag, possibly using a similar approach. > CC'ing Hao Ge to be in the loop of these changes. I'll work with him > on aliminating that __GFP_NO_OBJ_EXT alias. Good, then we can remove the flag completely. > [1] https://lore.kernel.org/all/20260604024008.46592-1-hao.ge@linux.dev/ > >> >> > >> > #ifdef CONFIG_SLAB_OBJ_EXT >> > >> > >> >> -- >> Thanks, >> Hao