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b=kK4GS7G8sLO37sfOkMRYMw9J2p2H3Czzi2BISfaOLZ4VFFZDHMuElpHz3CxwcWLXQ Iah53dkaEN9B1KoE0fVMfqb5/yU2366ZV1mh5Js2AG9ZViZP+P3tByC84K4wOFg5V6 2Pf4lNLdRUap9BqfHcF+d3Kwp/eFx1rrp6fk33mDeTus0s9CUlGhShqMObskb6Ob4a Qwlgdw7nnUwX3YWdhkB5Y3BTUvXjHb871K4vGoQwCBGU2wwCxMMPYxneoiodT6Ksd8 7LRrsQLtUMeL8zzl0C00Axql7TIRHl8UYtauBGHhIwqs/5MX6wh1XOoscD/Pb8Aenp 7haqKSYSYjQ1A== Message-ID: <6b6b094b-8dcb-423b-bb86-ef1439887eed@kernel.org> Date: Fri, 8 May 2026 23:41:34 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/khugepaged: clear MMF_VM_HUGEPAGE on mm_slot_alloc() failure To: Lance Yang , dev.jain@arm.com Cc: ye.liu@linux.dev, akpm@linux-foundation.org, ljs@kernel.org, xhao@linux.alibaba.com, liuye@kylinos.cn, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, akpm@linux-foudation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <3fb8c00d-d60d-4fec-81b1-e7633384a52d@arm.com> <20260506105150.21504-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 7A32680007 X-Stat-Signature: fut51iagdgpswf4itz9jug5n4g15srb5 X-Rspam-User: X-HE-Tag: 1778276503-769923 X-HE-Meta: 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 abOVmohs Y6UbqeZHWjkixeAIGufdZSth0pxoRgkOP/wD8SByNJ9iapG9kCoKx4yDL8xTlyRIlL4eypQrtNEIsZ87Wu/5ocTwrgBOXSzz6yyFrd7/144Nq5KAtn4ldPKSb3fIT29kQWjdaWHbdTr5x6omknKiVcYb2V65Vu+JYVkMCQGSzTzFhAYbvv7A7z+fKqmsfL3NoDN0W8QfSiy49oA06ZBdBVv1Pt6QoHGp2z8xTP3zehwEpO65QkoYWvAxoOIWOktD8W/eXj3IKmN2v3p8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/6/26 12:51, Lance Yang wrote: > > On Wed, May 06, 2026 at 12:16:35PM +0530, Dev Jain wrote: >> >> >> On 06/05/26 6:51 am, Ye Liu wrote: >>> From: Ye Liu >>> >>> __khugepaged_enter() sets MMF_VM_HUGEPAGE before allocating the >>> corresponding mm_slot. If mm_slot_alloc() fails, the function >>> returns with the flag set but without inserting the mm into the >>> khugepaged tracking structures. >>> >>> This leaves the mm in an inconsistent state: it is marked as >>> registered (MMF_VM_HUGEPAGE set), but will never be scanned by >>> khugepaged. Future attempts to register the mm are skipped since >>> khugepaged_enter_vma() checks the flag and returns early. >>> >>> Fix this by clearing MMF_VM_HUGEPAGE when mm_slot_alloc() fails, >>> restoring the ability to retry registration later. >>> >>> Fixes: 16618670276a ("mm: khugepaged: avoid pointless allocation for struct mm_slot") >>> Signed-off-by: Ye Liu >>> --- >>> Changes since v1: >>> - Add Fixes tag as suggested by Dev Jain and Lance Yang >>> >>> mm/khugepaged.c | 4 +++- >>> 1 file changed, 3 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 7d48d4fbd5f3..60ab7c1b61dd 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -559,8 +559,10 @@ void __khugepaged_enter(struct mm_struct *mm) >>> return; >>> >>> slot = mm_slot_alloc(mm_slot_cache); >>> - if (!slot) >>> + if (!slot) { >>> + mm_flags_clear(MMF_VM_HUGEPAGE, mm); >>> return; >>> + } >> >> Note that, a racing khugepaged_enter_vma() may back off >> when it sees that MMF_VM_HUGEPAGE is set, but then the above >> clears the flag after slot alloc failure. So we end up not >> registering the mm with khugepaged. But I am sure no one >> cares, we are in much big trouble if slot alloc is failing. > > Right. A racing khugepaged_enter_vma() can see MMF_VM_HUGEPAGE is set > and return, then !slot clears it again. If there is no later > khugepaged_enter_vma(), the mm still wouldn't get registered :) So why not diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 5f4e009593e0..78735f34250a 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -437,13 +437,16 @@ void __khugepaged_enter(struct mm_struct *mm) /* __khugepaged_exit() must not run from under us */ VM_BUG_ON_MM(collapse_test_exit(mm), mm); - if (unlikely(mm_flags_test_and_set(MMF_VM_HUGEPAGE, mm))) - return; slot = mm_slot_alloc(mm_slot_cache); if (!slot) return; + if (unlikely(mm_flags_test_and_set(MMF_VM_HUGEPAGE, mm))) { + mm_slot_free(mm_slot_cache, slot); + return; + } + spin_lock(&khugepaged_mm_lock); mm_slot_insert(mm_slots_hash, mm, slot); /* Arguably, on the race described above, likely the thread seeing the MMF_VM_HUGEPAGE would likely similarly have failed the allocation. I'm fine with either, just wanted to raise the (cleaner looking?) alternative where we just properly back off? -- Cheers, David