From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0FE1DCCF9EE for ; Tue, 28 Oct 2025 13:37:29 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 5F7E180147; Tue, 28 Oct 2025 09:37:29 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 57F368013F; Tue, 28 Oct 2025 09:37:29 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 421CC80147; Tue, 28 Oct 2025 09:37:29 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 271DB8013F for ; Tue, 28 Oct 2025 09:37:29 -0400 (EDT) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay02.hostedemail.com (Postfix) with ESMTP id E27F613B087 for ; Tue, 28 Oct 2025 13:37:28 +0000 (UTC) X-FDA: 84047624976.02.E183C2C Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf05.hostedemail.com (Postfix) with ESMTP id 8B45E100003 for ; Tue, 28 Oct 2025 13:37:26 +0000 (UTC) Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=U5LQPyNS; spf=pass (imf05.hostedemail.com: domain of npache@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=npache@redhat.com; dmarc=pass (policy=quarantine) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1761658646; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=391BPTjMq7U4Si7/L0Y5C310VmT6EKvmZL7j8j+OpFA=; b=b1U7IZr2cT0aOyhikczvUE0EfyLGRs4liyLgNa4J0fDdS9kA4O8vK7mg5Q3X/aLPOyBqRU uYem1eCbxSTzTKdgYEnRE//acR9PmZOW6dlNDioQVCeJFyv4NvSEgrXGpvvWkkeBL8X+dy 1/4jhL6lTVYOi2I2ElwcHc/gducJwa8= ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=U5LQPyNS; spf=pass (imf05.hostedemail.com: domain of npache@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=npache@redhat.com; dmarc=pass (policy=quarantine) header.from=redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1761658646; a=rsa-sha256; cv=none; b=xC5fhZH45dr0FTovVFvW5a5WkYzivs6rYV0b6ucgEuxfDFVXg7AyyChK8WL1T3VFHLnkyQ 0QTvvZ9bbLKs8KDCpw27FUtfbAWADCNdjigRpyUNJJ19Bwa0Uw748B+3NU/txBMl9SyckG MwNFNMWinoLa8G+WlEZx51KOAkNeL5M= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1761658645; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=391BPTjMq7U4Si7/L0Y5C310VmT6EKvmZL7j8j+OpFA=; b=U5LQPyNSOX7qdyvJia9PCLe5vWIuqJY21YwN0igFTOTah1vObfvqTzJxVNExH9VWGzAbQx aeyHJ7fZivL0VtbEin+bpdZnO0sQS2U8jqDWr2FEAOhcdkFVs4lklhgs81rfB0dQ77SoRc YihrePvL04P0iv3K2PTV1PJFROEmpQI= Received: from mail-yw1-f199.google.com (mail-yw1-f199.google.com [209.85.128.199]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-347-kc4VmQ_jNUO2CLdSlOR8tw-1; Tue, 28 Oct 2025 09:37:23 -0400 X-MC-Unique: kc4VmQ_jNUO2CLdSlOR8tw-1 X-Mimecast-MFC-AGG-ID: kc4VmQ_jNUO2CLdSlOR8tw_1761658643 Received: by mail-yw1-f199.google.com with SMTP id 00721157ae682-782c219f163so83192887b3.3 for ; Tue, 28 Oct 2025 06:37:23 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1761658643; x=1762263443; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=391BPTjMq7U4Si7/L0Y5C310VmT6EKvmZL7j8j+OpFA=; b=bPrNCeihQhT9/TCAJofKm3+rVnbr+rGWQLmggEyCmrXqxoysw7YAksmdx64Ps8gDjS xtHriEiwen5Z+iWdWC5ZmX/ls2fkUNhfxXVXYYXnFgWJgQCVsDoPcKHtzVQ8uG55Spxo VSiPkP/3c1dgkbQHr9tmRzNf1IAe/+TyZubWJmV+xmHz49ajOR9De9nWXD5Ql6VgfJEU /BH/j60gKsEBIydfGgLzS54qIlvl1XxydnYgr9rrMwwI9NZC7Bq6RyqJ2n7rx3djef8J k5InyJLagdVAA9YFNdfVWJJONooigXLqDjSnxsXxgczqpTOxEiN5L5SOch4uoy1GbdNT NQvg== X-Forwarded-Encrypted: i=1; AJvYcCVicdEvg/zcIJ8IYONz7hHaS0qCd96OaAgq401yuryXOMXl5Z2C2TwbxOi3S0prFhnz1TfmdmYq9Q==@kvack.org X-Gm-Message-State: AOJu0Yx3hPyPOmjhPntr+jWkgRLQQSvXkdD6RXMA+v74W5UiJIe1CCPa W9adj39Cubf3yqMMQatwBoy+UjEmgiKI2L4fZ4BNyzQd3yX5vaD42aqDqnP8PUC+GH1knMz94Pj 6kBwxlyYYmb+94x9+zQkPkfoYl9z5120+5+plRCxLGigFoCyjw4yVwDWmBNlqECzyxdJNcPlNLC Q9tW1WWq3ePw7COPxScalEBgansJM= X-Gm-Gg: ASbGncseUGx1QIldwNqEqmw+DFKEqlAH+y/1MMwLZ8SRAbzSauQxI80XqW4/aLQjNpP 0nYr33bNOGGC0xjwnTGH2wOjeRdPDbsGRz3R6IlUzTKC1B7l4OY4pfWUo/resHU9Ux86quGVgum 535fP//5t2B5LBx+oZxN26z+aapvntgUDtPpExDWopy2n58VAwusTrPl8rALkMr/F2oK4MzA== X-Received: by 2002:a53:cd41:0:b0:63c:f5a6:f2ef with SMTP id 956f58d0204a3-63f6bab5662mr2132917d50.65.1761658641282; Tue, 28 Oct 2025 06:37:21 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHI5+9/fHp4gwqVR+X5FCEMvrHe+GcwlBX4w2Wql+/7C/fl5yLGrv62o9V5yeHttxzVZEf22GwHT7NBFpcz7XM= X-Received: by 2002:a53:cd41:0:b0:63c:f5a6:f2ef with SMTP id 956f58d0204a3-63f6bab5662mr2132906d50.65.1761658640837; Tue, 28 Oct 2025 06:37:20 -0700 (PDT) MIME-Version: 1.0 References: <20251022183717.70829-1-npache@redhat.com> <20251022183717.70829-7-npache@redhat.com> <5f8c69c1-d07b-4957-b671-b37fccf729f1@lucifer.local> In-Reply-To: <5f8c69c1-d07b-4957-b671-b37fccf729f1@lucifer.local> From: Nico Pache Date: Tue, 28 Oct 2025 07:36:55 -0600 X-Gm-Features: AWmQ_bmW8XylmR-fGz7b8D2DLOax1IHPOfKa83653zAJ0iAfVZ2XGu7NC_3nK0A Message-ID: Subject: Re: [PATCH v12 mm-new 06/15] khugepaged: introduce collapse_max_ptes_none helper function To: Lorenzo Stoakes Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, dev.jain@arm.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, akpm@linux-foundation.org, baohua@kernel.org, willy@infradead.org, peterx@redhat.com, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kas@kernel.org, aarcange@redhat.com, raquini@redhat.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com, richard.weiyang@gmail.com, lance.yang@linux.dev, vbabka@suse.cz, rppt@kernel.org, jannh@google.com, pfalcato@suse.de X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: Sx65V21TPohkUIYgbdQU03OrMWr6WOh5ruvE6w5Yhlc_1761658643 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 8B45E100003 X-Stat-Signature: gr5gcjfg5cw6gy8jg4zqbbzhheqxjoqn X-Rspam-User: X-HE-Tag: 1761658646-398349 X-HE-Meta: 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 8lZR9XOY kyCDH6gFNMwX73K1s6yapFmrRJuhptLvZNSXRIqaJXBfFWZVtHZ3wx/RH2eYo+l4/NxlCePUKKzIBJ35bdqQO6kJoAbSYTu8BHeLEzCqNuyqmwqxPM5/2VYuuQdZU0WpPrpmIciha5vHEoyaUZnXP93ANfA9sQBWwrCYHawI31lCVebdpWpjTx5N63FYNroS4hjhLBvfv4B30ICiW27lBmcrfsxRh61lF2F/zlGtiySbtF5eGQdfwNcGTwQseT5GkOtjJqU0eaYhiL9MNuYZ8hh8cO6EI8z7nu9jWZkF2wcSjDBfu3dSwqDFDUnBTW0YL3BDASU+u1bVkPUcJHSH0Zta7bXp0YMx8Mu7SCZ0FAgjcO5Xg6gGMR71Q0Ll6Y7tPDx/YNNK55/FtcLQqOTeK7FnHt44rzPaOpzv8QfZ1wIlI8o8= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Oct 27, 2025 at 11:54=E2=80=AFAM Lorenzo Stoakes wrote: > > On Wed, Oct 22, 2025 at 12:37:08PM -0600, Nico Pache wrote: > > The current mechanism for determining mTHP collapse scales the > > khugepaged_max_ptes_none value based on the target order. This > > introduces an undesirable feedback loop, or "creep", when max_ptes_none > > is set to a value greater than HPAGE_PMD_NR / 2. > > > > 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 caps the max_ptes_no= ne > > to HPAGE_PMD_NR / 2 - 1 (255 on 4k page size). The function also scales > > the max_ptes_none number by the (PMD_ORDER - target collapse order). > > > > The limits can be ignored by passing full_scan=3Dtrue, this is useful f= or > > madvise_collapse (which ignores limits), or in the case of > > collapse_scan_pmd(), allows the full PMD to be scanned when mTHP > > collapse is available. > > > > Signed-off-by: Nico Pache > > --- > > mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++++++- > > 1 file changed, 34 insertions(+), 1 deletion(-) > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > index 4ccebf5dda97..286c3a7afdee 100644 > > --- a/mm/khugepaged.c > > +++ b/mm/khugepaged.c > > @@ -459,6 +459,39 @@ void __khugepaged_enter(struct mm_struct *mm) > > wake_up_interruptible(&khugepaged_wait); > > } > > > > +/** > > + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for c= ollapse > > + * @order: The folio order being collapsed to > > + * @full_scan: Whether this is a full scan (ignore limits) > > + * > > + * For madvise-triggered collapses (full_scan=3Dtrue), all limits are = bypassed > > + * and allow up to HPAGE_PMD_NR - 1 empty PTEs. > > + * > > + * For PMD-sized collapses (order =3D=3D HPAGE_PMD_ORDER), use the con= figured > > + * khugepaged_max_ptes_none value. > > + * > > + * For mTHP collapses, scale down the max_ptes_none proportionally to = the folio > > + * order, but caps it at HPAGE_PMD_NR/2-1 to prevent a collapse feedba= ck loop. > > + * > > + * Return: Maximum number of empty PTEs allowed for the collapse opera= tion > > + */ > > +static unsigned int collapse_max_ptes_none(unsigned int order, bool fu= ll_scan) > > +{ > > + unsigned int max_ptes_none; > > + > > + /* ignore max_ptes_none limits */ > > + if (full_scan) > > + return HPAGE_PMD_NR - 1; > > + > > + if (order =3D=3D HPAGE_PMD_ORDER) > > + return khugepaged_max_ptes_none; > > + > > + max_ptes_none =3D min(khugepaged_max_ptes_none, HPAGE_PMD_NR/2 - = 1); > Hey Lorenzo, > I mean not to beat a dead horse re: v11 commentary, but I thought we were= going > to implement David's idea re: the new 'eagerness' tunable, and again we'r= e now just > implementing the capping at HPAGE_PMD_NR/2 - 1 thing again? I spoke to David and he said to continue forward with this series; the "eagerness" tunable will take some time, and may require further considerations/discussion. > > I'm still really quite uncomfortable with us silently capping this value. > > If we're putting forward theoretical ideas that are to be later built upo= n, this > series should be an RFC. > > But if we really intend to silently ignore user input the problem is that= then > becomes established uAPI. > > I think it's _sensible_ to avoid this mTHP escalation problem, but the is= sue is > visibility I think. > > I think people are going to find it odd that you set it to something, but= then > get something else. The alternative solution is to not support max_ptes_none for mTHP collapse and not allow none/zero pages. This is essentially "capping" the value too. > > As an alternative we could have a new sysfs field: > > /sys/kernel/mm/transparent_hugepage/khugepaged/max_mthp_ptes_none > > That shows the cap clearly. > > In fact, it could be read-only... and just expose it to the user. That re= duces > complexity. I agree with Baolin here; adding another tunable will only increase the complexity for our future goals, and also provides needless insight into the internals when they can not be customized. Cheers, -- Nico > > We can then bring in eagerness later and have the same situation of > max_ptes_none being a parameter that exists (plus this additional read-on= ly > parameter). > > > + > > + return max_ptes_none >> (HPAGE_PMD_ORDER - order); > > + > > +} > > + > > void khugepaged_enter_vma(struct vm_area_struct *vma, > > vm_flags_t vm_flags) > > { > > @@ -546,7 +579,7 @@ static int __collapse_huge_page_isolate(struct vm_a= rea_struct *vma, > > pte_t *_pte; > > int none_or_zero =3D 0, shared =3D 0, result =3D SCAN_FAIL, refer= enced =3D 0; > > const unsigned long nr_pages =3D 1UL << order; > > - int max_ptes_none =3D khugepaged_max_ptes_none >> (HPAGE_PMD_ORDE= R - order); > > + int max_ptes_none =3D collapse_max_ptes_none(order, !cc->is_khuge= paged); > > > > for (_pte =3D pte; _pte < pte + nr_pages; > > _pte++, addr +=3D PAGE_SIZE) { > > -- > > 2.51.0 > > >