From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 9F34B54652 for ; Mon, 11 May 2026 04:01:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778472073; cv=none; b=sLMVTuIFJoak+6W2BFUW9SxIOmMvQ0EOsv0MhcjCvZZQbfCzUWmLqZj8AJ2C1nnkxxtBQJU1jrZz3NhDCLk96DoEMxPO/gFdGiNR2Heea7n3PR+ogw8GCXqEwuC7PG9Tsnf/Qbu4r53JwgULb0b4F8MFRDIV3C0uhW0mUtIuMDk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778472073; c=relaxed/simple; bh=Ze0K/dB4LHZYNGAyRbWeOVVY+VRDSzPHaSagb6AO+Ek=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=AbHbFrFXg9u90pZl0RH5ceUI2X845C3p72gj+hyR3SC7bzKsW6/EYNeqQXOztqJytNpljr8gN3xfFBixnepWYq4Symb0gSyD4gNiUhWiJnzhX53HIYs/z3XV0x2g+PTnOB8t0SqbK+s4bBYlXKyfrjAXx+tUrT4iU0CGZ6D1DTo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=B6yi5fEW; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="B6yi5fEW" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2C7D41713; Sun, 10 May 2026 21:01:04 -0700 (PDT) Received: from [10.164.148.37] (MacBook-Pro.blr.arm.com [10.164.148.37]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id AB4963F85F; Sun, 10 May 2026 21:01:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1778472069; bh=Ze0K/dB4LHZYNGAyRbWeOVVY+VRDSzPHaSagb6AO+Ek=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=B6yi5fEWqy8MWcTv8zi+90Kv9d8NeRJLHFRGZcn11QlHv0JTPwF7zuCmlgFNxV9Re fR8jHQQP4pY3+/KE43KPG5+7k68DhFSiazqBxXQhdRy5GTed23z2GCMdHO+jMqzv1g uEZhHCkbt1y1PqmxzH/ftuncw0x5YGcLU38lnq8I= Message-ID: <41676526-ff0f-4172-8871-0a825ab524d0@arm.com> Date: Mon, 11 May 2026 09:30:42 +0530 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: "David Hildenbrand (Arm)" , Lance Yang 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> <6b6b094b-8dcb-423b-bb86-ef1439887eed@kernel.org> Content-Language: en-US From: Dev Jain In-Reply-To: <6b6b094b-8dcb-423b-bb86-ef1439887eed@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 09/05/26 3:11 am, David Hildenbrand (Arm) wrote: > 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? Yes this is also fine - I am overthinking but I wasn't going this way because ... A process doing THP allocations will fail on the mm_flags_test_and_set everytime after the first time. So we will have an unconditional overhead of mm_slot_alloc in the fault handler. Again I am overthinking : ) >