From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B569C309EFF for ; Fri, 8 May 2026 21:41:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778276502; cv=none; b=fvIBtQNJOymWnqHAvBvoHoZEqTRr6V5m1y6+KWPUMtgpCvRJuWhtmQS1v0Ugqvu8QMkJSb57a+QEjev39C7SE3Zi4X7EyjWkLF5nK08kyUZzHHoxAuEC+3d2fdKKpoGIHbehvMfcJbBxdq1Xa7MsjDKWaAzLLbnKVX8jtxyYF5Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778276502; c=relaxed/simple; bh=C5uNZOk0+Ox0K/5DzoimGv/kGBAg9O/PngG4bk1Ydx0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=N4rc5sp5e6a9PilrWxmTp3YJgCrnPNRMALPPEDtwzPzeTyYifkb/u32IW0HfSkEfElHsBPciZ36B4+eLHFdikjW9iLy1VbtTCp38homuY31diQVzuuSnCsMXbjbmI8M7D+pOf6XHVK+mkYg9abrWsNa1yVVX3kWs7N+8jigA9rQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kK4GS7G8; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kK4GS7G8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B55FAC2BCB0; Fri, 8 May 2026 21:41:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778276502; bh=C5uNZOk0+Ox0K/5DzoimGv/kGBAg9O/PngG4bk1Ydx0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260506105150.21504-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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