From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 00A73332913; Fri, 22 May 2026 21:24:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779485082; cv=none; b=QNztUuHRIZMIZKM1nCJT5pzWIVsmUuWqQi3dBVDOOOCDd0Lqr86BVKjde38NJir8mWkJThYf+YHBisKEHK6pOq1k+oit193/aA7gJ05FEmtPmxNxiOlyau/TGj/qrHVamsLXxK3CJr+NcwjQrU4lNVXFOca2U7OXbHdrU/Ic0zw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779485082; c=relaxed/simple; bh=Gg2Rkrz/C3O5mNC1uC0BufDHhrM3QVEXTwgQ36XjnYU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UfSck91CskiHvdLzcrGUXmgxMiWb0ced2lTBIn+QZiAS/aZ3U6nFpgN5bMKjcRDIgnSsaV1CyaZWzhckzfzoinjVxrd6wz6MvImLI8cU/3J4IQX5bLwh3MMOz1UsXzfZRlcZZIomRmv5njZPGHcJgv1UVk2HS1TVRWot8l6PM3c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BqYg8XWu; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="BqYg8XWu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 981681F000E9; Fri, 22 May 2026 21:24:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779485080; bh=kj+AcNSaVbXFkwBDds8y0U5mnhTG0bXrSzeVbSwlnsU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=BqYg8XWuwX142KZEjOt0ILzzc1PvRWabcF/izUWJ72StQyRTaARj0b5SqqFH91qvg TOvD3RzjeZ2/is9H4DcAYd3Mb4KYH99GKWB1jgk4kj8oRj1Luz7iTUc8GqtTQtM4qW D62N9LUyD28ffxr86RTJFlxERkWkFTthLIlKaa4BlBTuwuzwfn5Tm9pmaJf/JUjur7 caNVtyNDzLL0IRQMuKuIixclP1QyuGybZkzagrH3CmxaSQHUgt7UA0XwWmyCkadIS8 ySVcY3hyKS6n1GMihcMkiiDdpZ/exsuU35/oHnxnsyMprbSSywSV1LKRjBOeoDb3lu 4t1Nfz+elKnOg== Message-ID: <6962d78e-ed39-4dca-b7f1-c531292727f2@kernel.org> Date: Fri, 22 May 2026 23:24:25 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v18 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support To: Nico Pache , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Cc: aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, liam@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com References: <20260522150009.121603-1-npache@redhat.com> <20260522150009.121603-5-npache@redhat.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 5/22/26 16:59, Nico Pache wrote: > generalize the order of the __collapse_huge_page_* and collapse_max_* > functions to support future mTHP collapse. > > The current mechanism for determining collapse with the > khugepaged_max_ptes_none value is not designed with mTHP in mind. This > raises a key design issue: if we support user defined max_pte_none values > (even those scaled by order), a collapse of a lower order can introduces > an feedback loop, or "creep", when max_ptes_none is set to a value greater > than HPAGE_PMD_NR / 2. [1] > > With this configuration, a successful collapse to order N will populate > enough pages to satisfy the collapse condition on order N+1 on the next > scan. This leads to unnecessary work and memory churn. > > To fix this issue introduce a helper function that will limit mTHP > collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1. > This effectively supports two modes: [2] > > - max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE > that maps the shared zeropage. Consequently, no memory bloat. > - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest > available mTHP order. > > This removes the possibility of "creep", and a warning will be emitted if > any non-supported max_ptes_none value is configured with mTHP enabled. > Any intermediate value will default mTHP collapse to max_ptes_none=0. > > mTHP collapse will not honor the khugepaged_max_ptes_shared or > khugepaged_max_ptes_swap parameters, and will fail if it encounters a > shared or swapped entry. > > No functional changes in this patch; however it defines future behavior > for mTHP collapse. > > [1] - https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com > [2] - https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com > > Reviewed-by: Lance Yang > Co-developed-by: Dev Jain > Signed-off-by: Dev Jain > Signed-off-by: Nico Pache > --- > mm/khugepaged.c | 121 +++++++++++++++++++++++++++++++++++------------- > 1 file changed, 88 insertions(+), 33 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 116f39518948..e98ba5b15163 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -353,30 +353,52 @@ static bool pte_none_or_zero(pte_t pte) > * the shared zeropage for the given collapse operation. > * @cc: The collapse control struct > * @vma: The vma to check for userfaultfd > + * @order: The folio order being collapsed to > * > * Return: Maximum number of empty/shared zeropage PTEs for the collapse operation > */ > static unsigned int collapse_max_ptes_none(struct collapse_control *cc, > - struct vm_area_struct *vma) > + struct vm_area_struct *vma, unsigned int order) > { > + unsigned int max_ptes_none = khugepaged_max_ptes_none; Can be const, right? > + > if (vma && userfaultfd_armed(vma)) > return 0; > /* for MADV_COLLAPSE, allow any empty/shared zeropage PTEs */ > if (!cc->is_khugepaged) > return HPAGE_PMD_NR; > - /* For all other cases respect the user defined maximum */ > - return khugepaged_max_ptes_none; > + /* for PMD collapse, respect the user defined maximum */ > + if (is_pmd_order(order)) > + return max_ptes_none; > + /* > + * for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT, > + * scale the maximum number of PTEs to the order of the collapse. > + */ > + if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT) > + return (1 << order) - 1; > + if (!max_ptes_none) > + return 0; > + /* > + * For mTHP collapse of values other than 0 or KHUGEPAGED_MAX_PTES_LIMIT, > + * emit a warning and return 0. > + */ > + pr_warn_once("mTHP collapse does not support max_ptes_none values" > + " other than 0 or %u, defaulting to 0.\n", > + KHUGEPAGED_MAX_PTES_LIMIT); > + return 0; This might read slightly clearer as /* * For mTHP ... */ if (max_ptes_none) pr_warn_once(...) return 0; IOW, have a single "return 0" label here and only special-case when to warn. Acked-by: David Hildenbrand (arm) -- Cheers, David