linux-kernel - Re: [PATCH 2/2] mm, util: account_locked_vm() does not hold mmap

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Date:   Thu, 30 Jul 2020 16:57:05 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Pengfei Li <fly@...nel.page>, akpm@...ux-foundation.org,
        bmt@...ich.ibm.com, dledford@...hat.com, willy@...radead.org,
        vbabka@...e.cz, kirill.shutemov@...ux.intel.com, jgg@...pe.ca,
        alex.williamson@...hat.com, cohuck@...hat.com,
        daniel.m.jordan@...cle.com, dbueso@...e.de, jglisse@...hat.com,
        jhubbard@...dia.com, ldufour@...ux.ibm.com,
        Liam.Howlett@...cle.com, peterz@...radead.org, cl@...ux.com,
        jack@...e.cz, rientjes@...gle.com, walken@...gle.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm, util: account_locked_vm() does not hold mmap_lock

On Wed, Jul 29, 2020 at 12:21:11PM -0700, Hugh Dickins wrote:
> On Sun, 26 Jul 2020, Pengfei Li wrote:
> 
> > Since mm->locked_vm is already an atomic counter, account_locked_vm()
> > does not need to hold mmap_lock.
> 
> I am worried that this patch, already added to mmotm, along with its
> 1/2 making locked_vm an atomic64, might be rushed into v5.9 with just
> that two-line commit description, and no discussion at all.
> 
> locked_vm belongs fundamentally to mm/mlock.c, and the lock to guard
> it is mmap_lock; and mlock() has some complicated stuff to do under
> that lock while it decides how to adjust locked_vm.
>
> It is very easy to convert an unsigned long to an atomic64_t, but
> "atomic read, check limit and do stuff, atomic add" does not give
> the same guarantee as holding the right lock around it all.

Yes, this is why I withdrew my attempt to do something similar last year, I
didn't want to make the accounting racy.  Stack and heap growing and mremap
would be affected in addition to mlock.

It'd help to hear more about the motivation for this.

Daniel