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b=sQDH5ZEMeaGsZ0xpIfLrJWKdCEwlD0IZWgIRRwmZ0UiFIbCzhilp4pEkhdjc91qX9 jhhby5L/HIWRcvHXT9EhsKiKEtLGzkhsL2tkDwrlOEsgAA7OS11fKOzj0o9SXkURLJ DUEoKIubJbc+/swSeSpYh4K0gmkW1lp5yzYPn50xpOQHadIEt7V8i1udQfX5yfOkYh B18Rfo3vQWi+oFrw/0Teh47Db16fllaRKeXIdrqJk6Oq2ApCXdGdNzZhS2g9qORLpm borhS0eUSee0GLZ13N7XyNAs74TXnV56mjc0ZzUP/1Kj8QnCRk8eCkW+maTH7yZnx0 NSubKZWlk6xfQ== Message-ID: <11e01f34-d042-4938-b7c5-328e08f17192@kernel.org> Date: Mon, 13 Apr 2026 09:37:54 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v15 06/13] mm/khugepaged: skip collapsing mTHP to smaller orders To: Nico Pache Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, 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.Howlett@oracle.com, lorenzo.stoakes@oracle.com, 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: <20260226031741.230674-1-npache@redhat.com> <20260226032445.233437-1-npache@redhat.com> <94290ad5-f63d-4fa7-a898-dcfe6cd9998b@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Stat-Signature: 4ot6y3s84erukr33qusf7uxtiapefxox X-Rspamd-Queue-Id: DF98B180004 X-Rspam-User: X-HE-Tag: 1776065891-442057 X-HE-Meta: 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 OW/MHDUP Dfy5ebPIuVndaslrfLgwEFTCr8VEtHlrwMCTY28Kg8FMQIKT47tlUGu21suK+boeHL7AIgi0J4wxVralTyVNIcO4OxkVLYzGd2D5cK3n4OqMqGCP973UFAVSpFPgPG+25dGYu6VbVfMmnMITVPIJh/LQP6yZU0COaae6p/KgWeSjqoR+VtndnGRLIowj3cMDQss3PgebUJKLQ+ZwjxN8Vp6qdlXWZEOrewOm/3bYTig4GyYsSyeShx1sVxAAIqLbuNLMl4WA7kInk0W6LKdTgS+SoEYGV+xaLQsE4cdDxRoyLh1xC8Srk08Q32wbuxARy5/5YMAAQQ42xKOTvp4rPEEBd9NcETVb8k6L8hjK0734CmL3UtaGNrk2XQ8j61sujvLXEeLdx/n0xNxpW1ffHIztBvI4n+sqMWK9FCnQTD5OVu9nZZV34D8Ma0ijWXaXnqrwPzr6brKQkZ8+mcXOQEswlxw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/13/26 03:38, Nico Pache wrote: > On Thu, Mar 12, 2026 at 3:00 PM David Hildenbrand (Arm) > wrote: >> >> On 2/26/26 04:24, Nico Pache wrote: >>> khugepaged may try to collapse a mTHP to a smaller mTHP, resulting in >>> some pages being unmapped. Skip these cases until we have a way to check >>> if its ok to collapse to a smaller mTHP size (like in the case of a >>> partially mapped folio). >>> >>> This patch is inspired by Dev Jain's work on khugepaged mTHP support [1]. >>> >>> [1] https://lore.kernel.org/lkml/20241216165105.56185-11-dev.jain@arm.com/ >>> >>> Reviewed-by: Lorenzo Stoakes >>> Reviewed-by: Baolin Wang >>> Co-developed-by: Dev Jain >>> Signed-off-by: Dev Jain >>> Signed-off-by: Nico Pache >>> --- >>> mm/khugepaged.c | 8 ++++++++ >>> 1 file changed, 8 insertions(+) >>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index fb3ba8fe5a6c..c739f26dd61e 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -638,6 +638,14 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma, >>> goto out; >>> } >>> } >>> + /* >>> + * TODO: In some cases of partially-mapped folios, we'd actually >>> + * want to collapse. >>> + */ >>> + if (!is_pmd_order(order) && folio_order(folio) >= order) { >>> + result = SCAN_PTE_MAPPED_HUGEPAGE; >>> + goto out; >>> + } >>> >>> if (folio_test_large(folio)) { >>> struct folio *f; >> >> Why aren't we doing the same in hpage_collapse_scan_pmd() ? > > We can't do this in the scan phase because we are not yet aware of the > order we want to collapse to. Yes, realized that myself later. It's confusing, try documenting that in the patch description. -- Cheers, David