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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=eN3JnKs3EzS0t8ox0F9OPSRe2IdraPZxQZz4lC5+FzUfJpVjaxcEi6Otz6sc0BiAv tiKjVgEAXkLleHeGlaRilLr8esdBsvELnrUCFeqTNFz4370OKxKOz040DwWBIBbXzN eMt/BlMsKJF7KgZkYOrnCXqjIpNQDJgGX+pOG0FM= Message-ID: Date: Fri, 15 May 2026 12:34:17 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/3] mm/slub: hw_tags: skip page-allocator unpoisoning on slab allocation To: Ryan Roberts , 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> <83aca9c5-6bd0-438e-b571-c4d0335c9901@arm.com> Content-Language: en-US From: Dev Jain In-Reply-To: <83aca9c5-6bd0-438e-b571-c4d0335c9901@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 2603840009 X-Rspamd-Server: rspam06 X-Stat-Signature: bk4rsdgwpp86mewoxinrjrcdqfxgoike X-HE-Tag: 1778828668-314955 X-HE-Meta: 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 O08YguID 797n5MGIoT517cNFrs9Sn+kKU5a4M1Rd2jGC7HaA7aJhC4VQ8nJJ4Ssz2Sd16Zoma9SnkEoqLKOmQPFi0f0MV2j8Rk473io2yqq/+B2+LRyMv7xkXJb35dLkiG+d9wRl4/cwRZSW3g6nxvw1hRaqPysJpbvnh7itTBFDXWgHQxpQmPOB7dgZbIKhR2e1LbD9itZXOZtJw6aw55nx4P7ZWVeI09Z7ql3hgVSz6XBqj0LgFAlQHfh58db5LFK0Wm1J0R6nooQ1vCbgGNdqQwK72dy/puhmEdhYiNOfeHAjrS0Sm4RjfAVeJBwCWYqsnxZgu4QQ1se85ZOtLAg3mwEHkDpbMCFfMsxk1rLcUtlFvKzqRW8xT5c6Zl/ilPzolBXkGOUdMe5v9+LcKnFfwdGcW/1PdlCgfDAFSbvYn Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 14/05/26 5:41 pm, Ryan Roberts wrote: > 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. Okay. > > 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. GFP_SKIP_KASAN is only for HW_TAGS. I will mention the KASAN_SLAB_REDZONE = KASAN_PAGE_FREE for hw tags in the description. > > 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? Yes I will keep this. > >> - 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 >