linux-kernel - Re: [PATCH] mm: Fix kernel stack tagging for certain configs

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Message-ID: <239ecae6-2fc0-43be-bc63-08f52b39a524@redhat.com>
Date: Thu, 4 Sep 2025 08:42:17 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>,
 "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>, Kees Cook <kees@...nel.org>,
 kernel test robot <lkp@...el.com>, Dan Carpenter <dan.carpenter@...aro.org>
Subject: Re: [PATCH] mm: Fix kernel stack tagging for certain configs

On 02.09.25 21:30, Matthew Wilcox wrote:
> On Tue, Sep 02, 2025 at 10:59:03AM -0700, Vishal Moola (Oracle) wrote:
>> There are 3 cases where kernel pages are allocated for kernel stacks:
>> CONFIG_VMAP_STACK, THREAD_SIZE >= PAGE_SIZE, THREAD_SIZE < PAGE_SIZE.
>> These cases use vmalloc(), alloc_pages() and kmem_cache_alloc()
>> respectively.
> 
> I missed that the third case existed ...
> 
>> In the first 2 cases, THREAD_SIZE / PAGE_SIZE will always be greater
>> than 0, and pages are tagged as expected. In the third case,
>> THREAD_SIZE / PAGE_SIZE evaluates to 0 and doesn't tag any pages at all.
>> This meant that in those configs, the stack tagging was a no-op, and led
>> to smatch build warnings.
> 
> I didn't see those smatch warnings.  Were they cc'd to the mailing list?
> 
>> We definitely have at least 1 page we want tagged at this point, so fix
>> it by using a do {} while loop instead of a for loop.
> 
> We can't do this.  Pages can only have one type at a time.  Since
> they're allocated from slab, they have PGTY_slab.  This will lead to
> a warning at runtime:
> 
>          VM_BUG_ON_PAGE(data_race(page->page_type) != UINT_MAX, page);   \
> 
> But for our purposes (trying to figure out how fragmented the vmap
> stack is making the memmap), we don't need to do this accounting.
> These pages are already being allocated from slab, and slab allocates
> naturally aligned, so we can skip all of this for these configurations.

IIUC what you mean, I am not a fan of having PGTY_stack being properly 
set or not based on a kernel config.

I much rather prefer this being done cleanly for all cases, or not at 
all (OOT for research/debugging purposes).

-- 
Cheers

David / dhildenb