linux-kernel - Re: [PATCH v3 00/11] Mitigate a vmap lock contention v3

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Date: Thu, 22 Feb 2024 23:15:59 +0000
From: Pedro Falcato <pedro.falcato@...il.com>
To: Uladzislau Rezki <urezki@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>, Mel Gorman <mgorman@...e.de>, 
	kirill.shutemov@...ux.intel.com, Vishal Moola <vishal.moola@...il.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, LKML <linux-kernel@...r.kernel.org>, 
	Baoquan He <bhe@...hat.com>, Lorenzo Stoakes <lstoakes@...il.com>, 
	Christoph Hellwig <hch@...radead.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Dave Chinner <david@...morbit.com>, "Paul E . McKenney" <paulmck@...nel.org>, 
	Joel Fernandes <joel@...lfernandes.org>, Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>, 
	linux-mm@...ck.org
Subject: Re: [PATCH v3 00/11] Mitigate a vmap lock contention v3

Hi,

On Thu, Feb 22, 2024 at 8:35 AM Uladzislau Rezki <urezki@...il.com> wrote:
>
> Hello, Folk!
>
>[...]
> pagetable_alloc - gets increased as soon as a higher pressure is applied by
> increasing number of workers. Running same number of jobs on a next run
> does not increase it and stays on same level as on previous.
>
> /**
>  * pagetable_alloc - Allocate pagetables
>  * @gfp:    GFP flags
>  * @order:  desired pagetable order
>  *
>  * pagetable_alloc allocates memory for page tables as well as a page table
>  * descriptor to describe that memory.
>  *
>  * Return: The ptdesc describing the allocated page tables.
>  */
> static inline struct ptdesc *pagetable_alloc(gfp_t gfp, unsigned int order)
> {
>         struct page *page = alloc_pages(gfp | __GFP_COMP, order);
>
>         return page_ptdesc(page);
> }
>
> Could you please comment on it? Or do you have any thought? Is it expected?
> Is a page-table ever shrink?

It's my understanding that the vunmap_range helpers don't actively
free page tables, they just clear PTEs. munmap does free them in
mmap.c:free_pgtables, maybe something could be worked up for vmalloc
too.
I would not be surprised if the memory increase you're seeing is more
or less correlated to the maximum vmalloc footprint throughout the
whole test.

-- 
Pedro