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From: Pengfei Li <fly@kernel.page>
To: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Hugh Dickins <hughd@google.com>,
	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, 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, fly@kernel.page
Subject: Re: [PATCH 2/2] mm, util: account_locked_vm() does not hold mmap_lock
Date: Sun, 2 Aug 2020 19:23:47 +0800
Message-ID: <20200802192347.534ece64.fly@kernel.page> (raw)
In-Reply-To: <20200730205705.ityqlyeswzo5dkow@ca-dmjordan1.us.oracle.com>

On Thu, 30 Jul 2020 16:57:05 -0400
Daniel Jordan <daniel.m.jordan@oracle.com> wrote:

> 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.
> 

Thanks for your comments.

My motivation is to allow mm related counters to be safely read and
written without holding mmap_lock. But sorry i didn't do well.

-- 
Pengfei

  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
2020-08-02 11:23       ` Pengfei Li [this message]
2020-08-02 11:07     ` Pengfei Li

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