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From: Pengfei Li <fly@kernel.page>
To: Hugh Dickins <hughd@google.com>
Cc: 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, fly@kernel.page
Subject: Re: [PATCH 2/2] mm, util: account_locked_vm() does not hold mmap_lock
Date: Sun, 2 Aug 2020 19:07:24 +0800
Message-ID: <20200802190724.493304b6.fly@kernel.page> (raw)
In-Reply-To: <alpine.LSU.2.11.2007291121280.4649@eggly.anvils>

On Wed, 29 Jul 2020 12:21:11 -0700 (PDT)
Hugh Dickins <hughd@google.com> wrote:

Sorry for the late reply.

> 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.
> 
> (At the very least, __account_locked_vm() in 1/2 should be changed to
> replace its atomic64_add by an atomic64_cmpxchg, to enforce the limit
> that it just checked.  But that will be no more than lipstick on a
> pig, when the right lock that everyone else agrees upon is not being
> held.)
> 

Thank you for your detailed comment.

You are right, I should use atomic64_cmpxchg to guarantee the limit of
RLIMIT_MEMLOCK.

> Now, it can be argued that our locked_vm and pinned_vm maintenance
> is so random and deficient, and too difficult to keep right across
> a sprawl of drivers, that we should just be grateful for those that
> do volunteer to subject themselves to RLIMIT_MEMLOCK limitation,
> and never mind if it's a little racy.
> 
> And it may well be that all those who have made considerable efforts
> in the past to improve the situation, have more interesting things to
> devote their time to, and would prefer not to get dragged back here.
> 
> But let's at least give this a little more visibility, and hope
> to hear opinions one way or the other from those who care.


Thank you. My patch should be more thoughtful.

I will send an email to Stephen soon asking to remove these two patches
from -mm tree.

-- 
Pengfei


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

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