linux-kernel - Re: [PATCH v3 00/15] Free user PTE page table pages

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Message-ID: <20211110174921.GA1740502@nvidia.com>
Date:   Wed, 10 Nov 2021 13:49:21 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Qi Zheng <zhengqi.arch@...edance.com>, akpm@...ux-foundation.org,
        tglx@...utronix.de, kirill.shutemov@...ux.intel.com,
        mika.penttila@...tfour.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        songmuchun@...edance.com, zhouchengming@...edance.com
Subject: Re: [PATCH v3 00/15] Free user PTE page table pages

On Wed, Nov 10, 2021 at 06:37:46PM +0100, David Hildenbrand wrote:

> It will be if we somewhat frequent when reclaim an empty PTE page table
> as soon as it turns empty.

What do you think is the frequency of unlocked page table walks that
this would have to block on?

> Also, it adds complexity for something that is only a problem in some
> corner cases -- sparse memory mappings, especially relevant for some
> memory allocators after freeing a lot of memory or running VMs with
> memory ballooning after inflating the balloon. Some of these use cases
> might be good with just triggering page table reclaim manually from user
> space.

Right, this is why it would be nice if the complexity could address
more than one problem, like the existing complex locking around the
thp stuff..

Jason