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smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0F367C4CEED; Thu, 21 Aug 2025 12:44:33 +0000 (UTC) Message-ID: <96148baf-f008-449b-988b-ea4f07d18528@redhat.com> Date: Thu, 21 Aug 2025 14:44:31 +0200 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] mm: tag kernel stack pages To: "Vishal Moola (Oracle)" , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Andrew Morton , Matthew Wilcox References: <20250820202029.1909925-1-vishal.moola@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20250820202029.1909925-1-vishal.moola@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 20.08.25 22:20, Vishal Moola (Oracle) wrote: > Currently, we have no way to distinguish a kernel stack page from an > unidentified page. Being able to track this information can be > beneficial for optimizing kernel memory usage (i.e. analyzing > fragmentation, location etc.). Knowing a page is being used for a kernel > stack gives us more insight about pages that are certainly immovable and > important to kernel functionality. It's a very niche use case. Anything that's not clearly a folio or a special movable_ops page is certainly immovable. So we can identify pretty reliable what's movable and what's not. Happy to learn how you would want to use that knowledge to reduce fragmentation. :) So this reads a bit hand-wavy. > > Add a new pagetype, and tag pages alongside the kernel stack accounting. > Also, ensure the type is dumped to /proc/kpageflags and the page-types > tool can find it. > > Signed-off-by: Vishal Moola (Oracle) > --- > fs/proc/page.c | 3 ++- > include/linux/page-flags.h | 5 +++++ > include/uapi/linux/kernel-page-flags.h | 1 + > kernel/fork.c | 19 +++++++++++++++++-- > tools/mm/page-types.c | 1 + > 5 files changed, 26 insertions(+), 3 deletions(-) > > diff --git a/fs/proc/page.c b/fs/proc/page.c > index 771e0b6bc630..46be207c5a02 100644 > --- a/fs/proc/page.c > +++ b/fs/proc/page.c > @@ -201,7 +201,8 @@ u64 stable_page_flags(const struct page *page) > > if (ps.flags & PAGE_SNAPSHOT_PG_BUDDY) > u |= 1 << KPF_BUDDY; > - > + if (folio_test_stack(folio)) > + u |= 1 << KPF_KSTACK; > if (folio_test_offline(folio)) > u |= 1 << KPF_OFFLINE; > if (folio_test_pgtable(folio)) > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index d53a86e68c89..5ee6ffbdbf83 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -933,6 +933,7 @@ enum pagetype { > PGTY_zsmalloc = 0xf6, > PGTY_unaccepted = 0xf7, > PGTY_large_kmalloc = 0xf8, > + PGTY_kstack = 0xf9, > > PGTY_mapcount_underflow = 0xff > }; > @@ -995,6 +996,10 @@ static __always_inline void __ClearPage##uname(struct page *page) \ > page->page_type = UINT_MAX; \ > } > > +/* PageStack() indicates that a page is used by kernel stacks. > + */ > +PAGE_TYPE_OPS(Stack, kstack, stack) > + > /* > * PageBuddy() indicates that the page is free and in the buddy system > * (see mm/page_alloc.c). > diff --git a/include/uapi/linux/kernel-page-flags.h b/include/uapi/linux/kernel-page-flags.h > index ff8032227876..56175b497ace 100644 > --- a/include/uapi/linux/kernel-page-flags.h > +++ b/include/uapi/linux/kernel-page-flags.h > @@ -36,5 +36,6 @@ > #define KPF_ZERO_PAGE 24 > #define KPF_IDLE 25 > #define KPF_PGTABLE 26 > +#define KPF_KSTACK 27 > > #endif /* _UAPILINUX_KERNEL_PAGE_FLAGS_H */ > diff --git a/kernel/fork.c b/kernel/fork.c > index 5115be549234..c8a6e1495acf 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -440,15 +440,22 @@ static void account_kernel_stack(struct task_struct *tsk, int account) > struct vm_struct *vm_area = task_stack_vm_area(tsk); > int i; > > - for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) > + for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) { > mod_lruvec_page_state(vm_area->pages[i], NR_KERNEL_STACK_KB, > account * (PAGE_SIZE / 1024)); > + __SetPageStack(vm_area->pages[i]); > + } > } else { > void *stack = task_stack_page(tsk); > + struct page *page = virt_to_head_page(stack); > + int i; > > /* All stack pages are in the same node. */ > mod_lruvec_kmem_state(stack, NR_KERNEL_STACK_KB, > account * (THREAD_SIZE / 1024)); > + > + for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++, page++) > + __SetPageStack(page); > } > } > > @@ -461,8 +468,16 @@ void exit_task_stack_account(struct task_struct *tsk) > int i; > > vm_area = task_stack_vm_area(tsk); > - for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) > + for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) { > memcg_kmem_uncharge_page(vm_area->pages[i], 0); > + __ClearPageStack(vm_area->pages[i]); > + } > + } else { > + struct page *page = virt_to_head_page(task_stack_page(tsk)); > + int i; > + > + for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++, page++) > + __ClearPageStack(page); > } > } Note that exit_task_stack_account() stack calls account_kernel_stack(tsk, -1), where you would do a non-senical __SetPageStack() first. ... so this would better be done in account_kernel_stack() based on the "int account" flag. But I wonder, if this should actually go to the actual place where we alloc/free. Now that it's no longer required to clear page types when freeing, alloc_thread_stack_node() might be a better place to set it, and to leave it set until freed. I'll leave Willy whether we actually want this type, cannot spot it under [1], but if we have sufficient types available, why not. BUT staring at [1], we allocate from vmalloc, so I would assume that these will be vmalloc-typed pages in the future and we cannot change the type later. [1] https://kernelnewbies.org/MatthewWilcox/Memdescs -- Cheers David / dhildenb