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

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Date: Fri, 23 Feb 2024 10:34:36 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Pedro Falcato <pedro.falcato@...il.com>
Cc: Uladzislau Rezki <urezki@...il.com>,
	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

On Thu, Feb 22, 2024 at 11:15:59PM +0000, Pedro Falcato wrote:
> 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.
>
Right. I see that for a user space, pgtables are removed. There was a
work on it.

>
> 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.
> 
Yes, the vmalloc footprint follows the memory usage. Some uses cases
map lot of memory.

Thanks for the input!

--
Uladzislau Rezki