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([2601:282:1c83:9aa0::72b3]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-88652b4aed6sm139789696d6.37.2025.12.03.13.02.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Dec 2025 13:02:18 -0800 (PST) Message-ID: Date: Wed, 3 Dec 2025 14:02:14 -0700 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 v13 mm-new 07/16] khugepaged: introduce collapse_max_ptes_none helper function From: Nico Pache To: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org Cc: david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.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, 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 References: <20251201174627.23295-1-npache@redhat.com> <20251201174627.23295-8-npache@redhat.com> Content-Language: en-US, en-ZM In-Reply-To: <20251201174627.23295-8-npache@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Andrew, The bot has reported a potential uninitialized use of a variable. Can you please squash the following fixup to this commit. Thank you, Nico ----8<---- >From 846f79d91a25ebad76cbab3690ae315cfe3cf278 Mon Sep 17 00:00:00 2001 From: Nico Pache Date: Wed, 3 Dec 2025 13:42:18 -0700 Subject: [PATCH] khugepaged: fixup unintialized _pte variable There is a potential use of an uninitialized variable after `khugepaged: introduce collapse_max_ptes_none helper function` Andrew can you please append this to Patch 7 of my series as reported by the kernel test robot >> mm/khugepaged.c:593:6: warning: variable '_pte' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized] 593 | if (max_ptes_none == -EINVAL) | ^~~~~~~~~~~~~~~~~~~~~~~~ mm/khugepaged.c:724:25: note: uninitialized use occurs here 724 | release_pte_pages(pte, _pte, compound_pagelist); | ^~~~ mm/khugepaged.c:593:2: note: remove the 'if' if its condition is always false 593 | if (max_ptes_none == -EINVAL) | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 594 | goto out; | ~~~~~~~~ mm/khugepaged.c:588:13: note: initialize the variable '_pte' to silence this warning 588 | pte_t *_pte; | ^ | = NULL 1 warning generated. Signed-off-by: Nico Pache --- mm/khugepaged.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index f425238d5d4f..7c7d04d6737e 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -585,7 +585,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, struct page *page = NULL; struct folio *folio = NULL; unsigned long addr = start_addr; - pte_t *_pte; + pte_t *_pte = pte; int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0; const unsigned long nr_pages = 1UL << order; int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged); @@ -593,8 +593,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, if (max_ptes_none == -EINVAL) goto out; - for (_pte = pte; _pte < pte + nr_pages; - _pte++, addr += PAGE_SIZE) { + for (; _pte < pte + nr_pages; _pte++, addr += PAGE_SIZE) { pte_t pteval = ptep_get(_pte); if (pte_none_or_zero(pteval)) { ++none_or_zero; -- 2.52.0 On 12/1/25 10:46 AM, 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 will limit mTHP > collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1. > This effectively supports two modes: > > - max_ptes_none=0: never introduce new none-pages for mTHP collapse. > - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest > available mTHP order. > > This removes the possiblilty of "creep", while not modifying any uAPI > expectations. A warning will be emitted if any non-supported > max_ptes_none value is configured with mTHP enabled. > > The limits can be ignored by passing full_scan=true, this is useful for > 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 | 43 ++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 42 insertions(+), 1 deletion(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 8dab49c53128..f425238d5d4f 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -463,6 +463,44 @@ void __khugepaged_enter(struct mm_struct *mm) > wake_up_interruptible(&khugepaged_wait); > } > > +/** > + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse > + * @order: The folio order being collapsed to > + * @full_scan: Whether this is a full scan (ignore limits) > + * > + * For madvise-triggered collapses (full_scan=true), all limits are bypassed > + * and allow up to HPAGE_PMD_NR - 1 empty PTEs. > + * > + * For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the configured > + * khugepaged_max_ptes_none value. > + * > + * For mTHP collapses, we currently only support khugepaged_max_pte_none values > + * of 0 or (HPAGE_PMD_NR - 1). Any other value will emit a warning and no mTHP > + * collapse will be attempted > + * > + * Return: Maximum number of empty PTEs allowed for the collapse operation > + */ > +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan) > +{ > + /* ignore max_ptes_none limits */ > + if (full_scan) > + return HPAGE_PMD_NR - 1; > + > + if (!is_mthp_order(order)) > + return khugepaged_max_ptes_none; > + > + /* Zero/non-present collapse disabled. */ > + if (!khugepaged_max_ptes_none) > + return 0; > + > + if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1) > + return (1 << order) - 1; > + > + pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %d\n", > + HPAGE_PMD_NR - 1); > + return -EINVAL; > +} > + > void khugepaged_enter_vma(struct vm_area_struct *vma, > vm_flags_t vm_flags) > { > @@ -550,7 +588,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, > pte_t *_pte; > int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0; > const unsigned long nr_pages = 1UL << order; > - int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order); > + int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged); > + > + if (max_ptes_none == -EINVAL) > + goto out; > > for (_pte = pte; _pte < pte + nr_pages; > _pte++, addr += PAGE_SIZE) {