linux-kernel - Re: [PATCH] mm: tag kernel stack pages

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Message-ID: <aLiGnSHFJdaTpMc-@fedora>
Date: Wed, 3 Sep 2025 11:19:09 -0700
From: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: tag kernel stack pages

On Wed, Sep 03, 2025 at 09:49:06AM +0200, David Hildenbrand wrote:
> [resending my original mail because it might have landed in the spam folder]

Ah, indeed the original mail was found in my spam folder. Thanks for
resending.

> 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.

My thoughts align with Matthew's response. If we decide "This doesn't add
enough value to merge it upstream" thats fine by me.

Otherwise if we think this is useful, I can respin this with your
suggestion below.

> 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 think this would be a better place to implement it as well.

> 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
>