linux-kernel - Re: [PATCH 3/3] mm/uffd: Detect pgtable allocation failures

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Message-Id: <832F027A-3778-4670-BED6-1222FA905F13@gmail.com>
Date:   Wed, 18 Jan 2023 13:51:30 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        James Houghton <jthoughton@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] mm/uffd: Detect pgtable allocation failures

Sorry for the late response.

>> Yes, I propose it as an optional flag for UFFD-WP.
>> Anyhow, I believe the UFFD-WP as implemented now is not efficient and
>> should’ve been vectored to allow one TLB shootdown for many
>> non-consecutive pages.
> 
> Agreed.  Would providing a vector of ranges help too for a few uffd ioctls?
> 
> I'm also curious whether you're still actively developing (or running) your
> iouring series.

So I finished building a prototype some time ago, and one of the benefits
was in reducing memory reclamation time. Unfortunately, MGLRU then came and
took away a lot of the benefit.

A colleague of mine had a slightly different use-case, so I gave him the
code and he showed interest in upstreaming it. After some probing, it turns
out he decided he is not into the effort of upstreaming it. I can upstream
the vectored WP once I write some tests.
>> 
>> I am not sure what the best way to detect that a page is write-pinned
>> reliably. My point was that if a change is already carried to
>> write-protect mechanisms, then this issue should be considered. Because
>> otherwise, many use-cases of uffd-wp would encounter implementation
>> issues.
>> 
>> I will not “kill” myself over it now, but I think it worth consideration.
> 
> The current interface change is small and limited only to the extra -ENOMEM
> retval with memory pressures (personally I don't really know how to trigger
> this, as I never succeeded myself even with memory pressure..).  What you
> said does sound like a new area to explore, and I think it's fine to change
> the interface again.

Understood.

Thanks and sorry again for the late response,
Nadav