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[2003:d8:2f30:ec00:8f7e:58a4:ebf0:6a36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-450064a1ba1sm15277355e9.16.2025.05.28.02.26.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 May 2025 02:26:28 -0700 (PDT) Message-ID: Date: Wed, 28 May 2025 11:26:25 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 06/12] khugepaged: introduce khugepaged_scan_bitmap for mTHP support To: Baolin Wang , Nico Pache , David Rientjes , zokeefe@google.com Cc: linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, 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, kirill.shutemov@linux.intel.com, 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, hannes@cmpxchg.org, mhocko@suse.com, rdunlap@infradead.org References: <20250515032226.128900-1-npache@redhat.com> <20250515032226.128900-7-npache@redhat.com> <9c54397f-3cbf-4fa2-bf69-ba89613d355f@linux.alibaba.com> <1f00fdc3-a3a3-464b-8565-4c1b23d34f8d@linux.alibaba.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 22.05.25 11:39, Baolin Wang wrote: > > > On 2025/5/21 18:23, Nico Pache wrote: >> On Tue, May 20, 2025 at 4:09 AM Baolin Wang >> wrote: >>> >>> Sorry for late reply. >>> >>> On 2025/5/17 14:47, Nico Pache wrote: >>>> On Thu, May 15, 2025 at 9:20 PM Baolin Wang >>>> wrote: >>>>> >>>>> >>>>> >>>>> On 2025/5/15 11:22, Nico Pache wrote: >>>>>> khugepaged scans anons PMD ranges for potential collapse to a hugepage. >>>>>> To add mTHP support we use this scan to instead record chunks of utilized >>>>>> sections of the PMD. >>>>>> >>>>>> khugepaged_scan_bitmap uses a stack struct to recursively scan a bitmap >>>>>> that represents chunks of utilized regions. We can then determine what >>>>>> mTHP size fits best and in the following patch, we set this bitmap while >>>>>> scanning the anon PMD. A minimum collapse order of 2 is used as this is >>>>>> the lowest order supported by anon memory. >>>>>> >>>>>> max_ptes_none is used as a scale to determine how "full" an order must >>>>>> be before being considered for collapse. >>>>>> >>>>>> When attempting to collapse an order that has its order set to "always" >>>>>> lets always collapse to that order in a greedy manner without >>>>>> considering the number of bits set. >>>>>> >>>>>> Signed-off-by: Nico Pache >>>>> >>>>> Sigh. You still haven't addressed or explained the issues I previously >>>>> raised [1], so I don't know how to review this patch again... >>>> Can you still reproduce this issue? >>> >>> Yes, I can still reproduce this issue with today's (5/20) mm-new branch. >>> >>> I've disabled PMD-sized THP in my system: >>> [root]# cat /sys/kernel/mm/transparent_hugepage/enabled >>> always madvise [never] >>> [root]# cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled >>> always inherit madvise [never] >>> >>> And I tried calling madvise() with MADV_COLLAPSE for anonymous memory, >>> and I can still see it collapsing to a PMD-sized THP. >> Hi Baolin ! Thank you for your reply and willingness to test again :) >> >> I didn't realize we were talking about madvise collapse-- this makes >> sense now. I also figured out why I could "reproduce" it before. My >> script was always enabling the THP settings in two places, and I only >> commented out one to test this. But this time I was doing more manual >> testing. >> >> The original design of madvise_collapse ignores the sysfs and >> collapses even if you have an order disabled. I believe this behavior >> is wrong, but by design. I spent some time playing around with madvise >> collapses with and w/o my changes. This is not a new thing, I >> reproduced the issue in 6.11 (Fedora 41), and I think its been >> possible since the inception of madvise collapse 3 years ago. I >> noticed a similar behavior on one of my RFC since it was "breaking" >> selftests, and the fix was to reincorporate this broken sysfs >> behavior. > > OK. Thanks for the explanation. > >> 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse") >> "This call is independent of the system-wide THP sysfs settings, but >> will fail for memory marked VM_NOHUGEPAGE." >> >> The second condition holds true (and fails for VM_NOHUGEPAGE), but I >> dont know if we actually want madvise_collapse to be independent of >> the system-wide. > > This design principle surprised me a bit, and I failed to find the > reason in the commit log. I agree that "never should mean never," and we > should respect the THP/mTHP sysfs setting. Additionally, for the > 'shmem_enabled' sysfs interface controlled for shmem/tmpfs, THP collapse > can still be prohibited through the 'deny' configuration. The rules here > are somewhat confusing. I recall that we decided to overwrite "VM_NOHUGEPAGE", because the assumption is that the same app that triggered MADV_NOHUGEPAGE triggers the collapse. So the app decides on its own behavior. Similarly, allowing for collapsing in a VM without VM_HUGEPAGE in the "madvise" mode would be fine. But in the "never" case, we should just "never" collapse. -- Cheers, David / dhildenb