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Message-ID: <96148baf-f008-449b-988b-ea4f07d18528@redhat.com>
Date: Thu, 21 Aug 2025 14:44:31 +0200
From: David Hildenbrand <david@...hat.com>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm: tag kernel stack pages
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) <vishal.moola@...il.com>
> ---
> 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
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