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 EF53E318EDC for ; Mon, 11 May 2026 05:40:51 +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=1778478052; cv=none; b=rLGdIujKSppEzzHfPIZKKSDXHIAlwttWQkdTRLB3J1BP0XE/rTWdPNz3O8e1ZWjkEooOFoBBuX+1x2936t9YNYYASmNRgDPUeL4kdElZ16/MpVrCVRga67MMJ35e51xghu2ApfBaZqcvo2G0D9BoL+E9CPwauVVh+Yc6Mb95+Qg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778478052; c=relaxed/simple; bh=JIyxTvGVC3zuQ2OR4m1RLr1N+pOyNiKOk9VaIsgzW3c=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rH+neMz34VlmteXfRavnLYBFE0FNBm+yOw9ONEa5w5kuVozxFyZJcWK6zhh6KCz48Z7HsUSJl5NA5d/BxXYtn6toM2mJZ9SvivXgN1l8zCyacNL0fWleus9PfRiL5ARXyn5eo1maCf3OHZW9Cz41Im5hbg5U2F/LfInNGbDMsDY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Kn6Va6ga; 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="Kn6Va6ga" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9D68BC2BCB0; Mon, 11 May 2026 05:40:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778478051; bh=JIyxTvGVC3zuQ2OR4m1RLr1N+pOyNiKOk9VaIsgzW3c=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Kn6Va6gaec/W9fP6O8mMug/XPZaH5PS9TrGRZcK3cd2xMyB+VSvEUQkJoExtgw9Dy SDgrzurscpRByuAHL/+6eMBSNK6priS2OThUPdXD1aUYNCTYu6qaHDwpf3i2iVb3NP 6PD4M0YlfyFMRqmujtVvnaXwKBDAjRktgVmTKAV26DrSv96KLaEu+qqxQYfYUwWLyt EmP45h9zrH3r3YupD0MOdvpQGIi6wXKJHYin3eFQVWNOwrQglMHqluruNqK7Boxy/g GtsQaSi5mIDf01MAE5WnV5iboaVjGQxn2HbCq/0qyGw7A5S5FkfGj6D07YwMv7kVXT 1A8LksMFlApRA== Message-ID: <5db3ccc6-7184-4068-918f-72dbfc31a781@kernel.org> Date: Mon, 11 May 2026 07:40:46 +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: Dev Jain , 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> <41676526-ff0f-4172-8871-0a825ab524d0@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/11/26 06:00, Dev Jain wrote: > > > On 09/05/26 3:11 am, David Hildenbrand (Arm) wrote: >> On 5/6/26 12:51, Lance Yang wrote: >>> >>> >>> 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. We should perform a mm_flags_test(MMF_VM_HUGEPAGE, vma->vm_mm) test before calling the function when the flag might not be set yet: in khugepaged_enter_vma() khugepaged_fork() should only get called once per process. Which makes sense, because mm_flags_test_and_set() is expensive. -- Cheers, David