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

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Date:   Wed, 10 Nov 2021 08:56:01 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     akpm@...ux-foundation.org, tglx@...utronix.de,
        kirill.shutemov@...ux.intel.com, mika.penttila@...tfour.com,
        david@...hat.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 06:54:13PM +0800, Qi Zheng wrote:

> In this patch series, we add a pte_refcount field to the struct page of page
> table to track how many users of PTE page table. Similar to the mechanism of
> page refcount, the user of PTE page table should hold a refcount to it before
> accessing. The PTE page table page will be freed when the last refcount is
> dropped.

So, this approach basically adds two atomics on every PTE map

If I have it right the reason that zap cannot clean the PTEs today is
because zap cannot obtain the mmap lock due to a lock ordering issue
with the inode lock vs mmap lock.

If it could obtain the mmap lock then it could do the zap using the
write side as unmapping a vma does.

Rather than adding a new "lock" to ever PTE I wonder if it would be
more efficient to break up the mmap lock and introduce a specific
rwsem for the page table itself, in addition to the PTL. Currently the
mmap lock is protecting both the vma list and the page table.

I think that would allow the lock ordering issue to be resolved and
zap could obtain a page table rwsem.

Compared to two atomics per PTE this would just be two atomic per
page table walk operation, it is conceptually a lot simpler, and would
allow freeing all the page table levels, not just PTEs.

?

Jason