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b=MymxaKhXa+HFQi0CPc2tUktfIJpbR1Wu0TshbMSBysMgG3qFaLtnQgFsxVLeTETwWqKA7clzfYWcK+0YzaBSflfKuf9IUxIhg+VsiUJj7S5YrLeF1JqPPCqgyawF6sTSIuUuX69CvwwX+0ehN0P6++vs0Pwty5DNWE0pVwAiSew= Received: from 30.74.144.118(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0Wv1EBZW_1765935235 cluster:ay36) by smtp.aliyun-inc.com; Wed, 17 Dec 2025 09:33:56 +0800 Message-ID: <45d61f3c-b721-4e42-8aa6-5f252193d2b7@linux.alibaba.com> Date: Wed, 17 Dec 2025 09:33:55 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v13 mm-new 07/16] khugepaged: introduce collapse_max_ptes_none helper function To: Nico Pache 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, 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, 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 References: <20251201174627.23295-1-npache@redhat.com> <20251201174627.23295-8-npache@redhat.com> <95b1403f-3ddb-43ff-b481-2ecc6ab8352f@linux.alibaba.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: D9C0016000A X-Stat-Signature: 6mupx44gz7gsybtj4aw7kgw9epdr333c X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1765935241-591879 X-HE-Meta: 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 RIeJIhq4 76SA3qm+59AHCGJZKhHvnwCeaDoyVFKNdVGVKteNS/jYJFRdWxBFS2YAHGiwa5Xz4R3QDaVqUZ0qUj3x/y/LvDrJ48PiG2327FE52AysseffKgx+wDOC94Tv4moNJRDNQy22jdWYrNxu9QW5l8JgogXx3Hyzbx7EdahJ9JUGf7euV5PvUCaTHxxSGwd3nFChYGIEil62slTt2VMdDoow0g+kh4oueeYj4cOu/zcTle8TdecTOBIMmefei021HBHHo8gcyd98dpp56iR90DuPfRLdr7Q+HTf7/4xRnAR/oIqrvwjvFE7YQK+LnNUCJubI3KMfA+PAsj6rsOKzgeG4nDXEUfjodPq7ChAqzHo2UkqqDNmnMiT5xJ5q5WGO4HsbUlrRaQFEwIqU+Z+GyrwE3iFzZbmi+SmuDUpHVrZH8dwcQchWzvtcrr6XHNba87BOraNBuy2E5+YurdpKFk82j2sAMkA== 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 2025/12/17 07:26, Nico Pache wrote: > On Tue, Dec 16, 2025 at 1:12 AM Baolin Wang > wrote: >> >> Hi Nico, > > Hi Baolin! Thanks for testing :) > > Did you happen to test with the changes I asked Andrew to append to > this commit? > > Either way, I think your fixup makes more sense than mine. Ah, I did not notice your fixup commit earlier, which seems to address this problem. And as I said, we don't need to call release_pte_pages() in this case. >> On 2025/12/2 01:46, 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; >> >> After testing your patchset, I hit the following crash. The reason is >> that when 'max_ptes_none' is -EINVAL here, it shouldn't goto out to call >> release_pte_pages(), because the '_pte' hasn't been initialized at this >> point, and there's no need to release folios either. >> >> After applying the fix below, the crash issue is resolved. I'm not sure >> whether Andrew will help fix this or if you will send a new version to >> address this issue. >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 8cffaf59ced8..2e8171a6d7df 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -646,7 +646,7 @@ static int __collapse_huge_page_isolate(struct >> vm_area_struct *vma, >> int max_ptes_none = collapse_max_ptes_none(order, >> !cc->is_khugepaged); >> >> if (max_ptes_none == -EINVAL) >> - goto out; >> + return result; >> >> for (_pte = pte; _pte < pte + nr_pages; >> _pte++, addr += PAGE_SIZE) { >> >> " >> [ 565.319345] Unable to handle kernel paging request at virtual address >> fffffffffffffffa >> ....... >> [ 565.319409] swapper pgtable: 4k pages, 48-bit VAs, pgdp=0000001f8549a000 >> [ 565.319416] [fffffffffffffffa] pgd=0000001f85f2a403, >> p4d=0000001f85f2a403, pud=0000001f85f2b403, pmd=0000000000000000 >> [ 565.319427] Internal error: Oops: 0000000096000006 [#1] SMP >> ....... >> [ 565.326733] pc : release_pte_pages+0x68/0x178 >> [ 565.326960] lr : __collapse_huge_page_isolate+0xc0/0x748 >> [ 565.327232] sp : ffff800083593910 >> ....... >> [ 565.331476] Call trace: >> [ 565.331664] release_pte_pages+0x68/0x178 (P) >> [ 565.331940] __collapse_huge_page_isolate+0xc0/0x748 >> [ 565.332249] collapse_huge_page+0x4cc/0xa70 >> [ 565.332510] mthp_collapse+0x254/0x2a8 >> [ 565.332754] collapse_scan_pmd+0x5a0/0x6d8 >> [ 565.333010] collapse_single_pmd+0x214/0x288 >> [ 565.333275] collapse_scan_mm_slot.constprop.0+0x2ac/0x460 >> [ 565.333617] khugepaged+0x204/0x2c8 >> [ 565.333992] kthread+0xf8/0x110 >> [ 565.334368] ret_from_fork+0x10/0x20 >> " >> >>> >>> for (_pte = pte; _pte < pte + nr_pages; >>> _pte++, addr += PAGE_SIZE) { >>