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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=gTpk+01HsJx+SY4XXRi+TfeWLmdtAaSc65TzL+eTZf8NWO1kNgixKc++/RhiXQ/hj RaRszNC/9gBBRo/qgd56UQk1Qx+0vcMYHzFvZ5X8dHcQMmjJc8Bf+s0ooqMA1+1wgw Q3vN42Zhz5yVKfRwP5cYJzQczIfFIQKG5Q8JZbHw= Message-ID: <83aca9c5-6bd0-438e-b571-c4d0335c9901@arm.com> Date: Thu, 14 May 2026 13:11:15 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/3] mm/slub: hw_tags: skip page-allocator unpoisoning on slab allocation Content-Language: en-GB To: Dev Jain , akpm@linux-foundation.org, vbabka@kernel.org, harry@kernel.org, ryabinin.a.a@gmail.com Cc: surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, hao.li@linux.dev, cl@gentwo.org, rientjes@google.com, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, glider@google.com, andreyknvl@gmail.com, dvyukov@google.com, vincenzo.frascino@arm.com, kasan-dev@googlegroups.com, anshuman.khandual@arm.com, catalin.marinas@arm.com References: <20260513105734.3380544-1-dev.jain@arm.com> <20260513105734.3380544-2-dev.jain@arm.com> From: Ryan Roberts In-Reply-To: <20260513105734.3380544-2-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 2020EA0009 X-Stat-Signature: xm5cfomy5663jwcgipqr8c9cnrce1rqi X-HE-Tag: 1778760681-881517 X-HE-Meta: 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 28ws+QS0 z7D5TbQWrtSrIEmfGvj1D498qjgMPWwUI7JPg3C/A+rcBy3X4u/MSF/q8luGks7y8tFJZMp73wxA0yIDGGyrmg9W0wEQerOUDNS7t6G++yCNj1md4RPK9bk6ruafIzvQyz26x394wDV/TpZwJ/ETUOqlJZZPGNw97+dehTCutbAP5wa7+DwScpk6wlqzhNg3Dzh0gxZLItwlE4uQbqvH6R0F/rFs7jTFIXj4RJ7qQrQ7tsK15Wi4EbSJ75k6PckXDqWICm+dvx7Ud+8am/dmNTHkAHyTeMizp830GaqfdMLvpuQOKu2ospUM9k1zhV9jWLAlVoQSLRFm/RCQpdkR24ri43J8IXWea/+x1HEMQgPx6lcFLkXdR2gQe/lFQdbPQDQw/RZF6sTbR/scbE/0QQn9wo4q66c0z47lg Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 13/05/2026 11:57, Dev Jain wrote: > When a new slab page is allocated, the buddy will unpoison the page. > Then slab immediately poisons the page via kasan_poison_slab(). This > is wasted work. > > Similar to what is done in vmalloc currently, use GFP_SKIP_KASAN > (hw tags flag only) to skip unpoisoning of the slab page. > > Signed-off-by: Dev Jain > --- > mm/page_alloc.c | 2 +- > mm/slub.c | 11 +++++++++-- > 2 files changed, 10 insertions(+), 3 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 227d58dc3de6..c3a69913aaa9 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -7723,7 +7723,7 @@ struct page *alloc_frozen_pages_nolock_noprof(gfp_t gfp_flags, int nid, unsigned > struct alloc_context ac = { }; > struct page *page; > > - VM_WARN_ON_ONCE(gfp_flags & ~__GFP_ACCOUNT); > + VM_WARN_ON_ONCE(gfp_flags & ~(__GFP_ACCOUNT | __GFP_SKIP_KASAN)); > /* > * In PREEMPT_RT spin_trylock() will call raw_spin_lock() which is > * unsafe in NMI. If spin_trylock() is called from hard IRQ the current > diff --git a/mm/slub.c b/mm/slub.c > index 0baa906f39ab..da3520769d1f 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -3269,9 +3269,16 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node, > struct slab *slab; > unsigned int order = oo_order(oo); > > + /* > + * New slab pages are immediately poisoned by kasan_poison_slab() > + * before any object is handed out, so page allocator unpoisoning > + * is wasted work for HW_TAGS KASAN. > + */ > + flags |= __GFP_SKIP_KASAN; You will also want to elide kasan_poison_slab() right? In which case, it might be better to handle __GFP_SKIP_KASAN in allocate_slab() (which calls alloc_slab_page()), because that's the place where kasan_poison_slab() currently is and it's probably better to keep the logic together. Note that there is a wrinkle though; logically, there are different types of memory poison; KASAN_PAGE_FREE, KASAN_PAGE_REDZONE, KASAN_SLAB_REDZONE, etc. Memory returned by the page allocator with __GFP_SKIP_KASAN set, will have KASAN_PAGE_FREE poison (I think). But kasan_poison_slab() sets KASAN_SLAB_REDZONE poison. However, this is only distinguished in practice for KASAN_GENERIC. For KASAN_SW_TAGS and KASAN_HW_TAGS these distinct logical types all map to the same KASAN_TAG_INVALID tag. So this optimization can only be safely applied to KASAN_SW_TAGS and KASAN_HW_TAGS. It would be nice if this could be abstracted away somehow... > + > if (unlikely(!allow_spin)) > - page = alloc_frozen_pages_nolock(0/* __GFP_COMP is implied */, nit: you may want to keep this comment around? > - node, order); > + page = alloc_frozen_pages_nolock(__GFP_SKIP_KASAN, > + node, order); > else if (node == NUMA_NO_NODE) > page = alloc_frozen_pages(flags, order); > else