linux-kernel - Re: [RFC PATCH RESEND 00/28] per-VMA locks proposal

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Message-ID: <CAJuCfpG3bMLzNhP5wt8my8j7_9wW=darLegd6WPV6tddtCKGAA@mail.gmail.com>
Date:   Mon, 5 Sep 2022 11:32:48 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michel Lespinasse <michel@...pinasse.org>,
        Jerome Glisse <jglisse@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>,
        Davidlohr Bueso <dave@...olabs.net>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R. Howlett" <liam.howlett@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Laurent Dufour <laurent.dufour@...ibm.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Song Liu <songliubraving@...com>, Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>, dhowells@...hat.com,
        Hugh Dickins <hughd@...gle.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Kent Overstreet <kent.overstreet@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Minchan Kim <minchan@...gle.com>,
        kernel-team <kernel-team@...roid.com>,
        linux-mm <linux-mm@...ck.org>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH RESEND 00/28] per-VMA locks proposal

On Mon, Sep 5, 2022 at 5:32 AM 'Michal Hocko' via kernel-team
<kernel-team@...roid.com> wrote:
>
> Unless I am missing something, this is not based on the Maple tree
> rewrite, right? Does the change in the data structure makes any
> difference to the approach? I remember discussions at LSFMM where it has
> been pointed out that some issues with the vma tree are considerably
> simpler to handle with the maple tree.

Correct, this does not use the Maple tree yet but once Maple tree
transition happens and it supports RCU-safe lookups, my code in
find_vma_under_rcu() becomes really simple.

>
> On Thu 01-09-22 10:34:48, Suren Baghdasaryan wrote:
> [...]
> > One notable way the implementation deviates from the proposal is the way
> > VMAs are marked as locked. Because during some of mm updates multiple
> > VMAs need to be locked until the end of the update (e.g. vma_merge,
> > split_vma, etc).
>
> I think it would be really helpful to spell out those issues in a greater
> detail. Not everybody is aware of those vma related subtleties.

Ack. I'll expand the description of the cases when multiple VMAs need
to be locked in the same update. The main difficulties are:
1. Multiple VMAs might need to be locked within one
mmap_write_lock/mmap_write_unlock session (will call it an update
transaction).
2. Figuring out when it's safe to unlock a previously locked VMA is
tricky because that might be happening in different functions and at
different call levels.

So, instead of the usual lock/unlock pattern, the proposed solution
marks a VMA as locked and provides an efficient way to:
1. Identify locked VMAs.
2. Unlock all locked VMAs in bulk.

We also postpone unlocking the locked VMAs until the end of the update
transaction, when we do mmap_write_unlock. Potentially this keeps a
VMA locked for longer than is absolutely necessary but it results in a
big reduction of code complexity.

>
> Thanks for working on this Suren!

Thanks for reviewing!
Suren.

> --
> Michal Hocko
> SUSE Labs
>
> --
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