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From: Daniel Jordan <daniel.m.jordan@oracle.com>
To: Hugh Dickins <hughd@google.com>
Cc: Pengfei Li <fly@kernel.page>,
	akpm@linux-foundation.org, bmt@zurich.ibm.com,
	dledford@redhat.com, willy@infradead.org, vbabka@suse.cz,
	kirill.shutemov@linux.intel.com, jgg@ziepe.ca,
	alex.williamson@redhat.com, cohuck@redhat.com,
	daniel.m.jordan@oracle.com, dbueso@suse.de, jglisse@redhat.com,
	jhubbard@nvidia.com, ldufour@linux.ibm.com,
	Liam.Howlett@oracle.com, peterz@infradead.org, cl@linux.com,
	jack@suse.cz, rientjes@google.com, walken@google.com,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH 2/2] mm, util: account_locked_vm() does not hold mmap_lock
Date: Thu, 30 Jul 2020 16:57:05 -0400
Message-ID: <20200730205705.ityqlyeswzo5dkow@ca-dmjordan1.us.oracle.com> (raw)
In-Reply-To: <alpine.LSU.2.11.2007291121280.4649@eggly.anvils>

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

  reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-26  8:02 [PATCH 1/2] mm: make mm->locked_vm an atomic64 counter Pengfei Li
2020-07-26  8:02 ` [PATCH 2/2] mm, util: account_locked_vm() does not hold mmap_lock Pengfei Li
2020-07-29 19:21   ` Hugh Dickins
2020-07-30 20:57     ` Daniel Jordan [this message]
2020-08-02 11:23       ` Pengfei Li
2020-08-02 11:07     ` Pengfei Li

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