Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Hugh Dickins <hughd@google.com>,
	Peter Feiner <pfeiner@google.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Pavel Emelyanov <xemul@parallels.com>,
	Jamie Liu <jamieliu@google.com>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Magnus Damm <magnus.damm@gmail.com>
Subject: Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared
Date: Tue, 26 Aug 2014 17:56:12 +0300	[thread overview]
Message-ID: <20140826145612.GA11226@node.dhcp.inet.fi> (raw)
In-Reply-To: <20140826141914.GA8952@moon>

On Tue, Aug 26, 2014 at 06:19:14PM +0400, Cyrill Gorcunov wrote:
> On Tue, Aug 26, 2014 at 05:04:19PM +0300, Kirill A. Shutemov wrote:
> > > > 
> > > > But now I'm realizing that if this is the _only_ place which modifies
> > > > vm_flags with down_read, then it's "probably" safe.  I've a vague
> > > > feeling that this was discussed before - is that so, Cyrill?
> > > 
> > > Well, as far as I remember we were not talking before about vm_flags
> > > and read-lock in this function, maybe it was on some unrelated lkml thread
> > > without me CC'ed? Until I miss something obvious using read-lock here
> > > for vm_flags modification should be safe, since the only thing which is
> > > important (in context of vma-softdirty) is the vma's presence. Hugh,
> > > mind to refresh my memory, how long ago the discussion took place?
> > 
> > It seems safe in vma-softdirty context. But if somebody else will decide that
> > it's fine to modify vm_flags without down_write (in their context), we
> > will get trouble. Sasha will come with weird bug report one day ;)
> > 
> > At least vm_flags must be updated atomically to avoid race in middle of
> > load-modify-store.
> 
> Which race you mean here? Two concurrent clear-refs?

Two concurent clear-refs is fine. But if somebody else will exploit the
same approch to set/clear other VM_FOO and it will race with clear-refs
we get trouble: some modifications can be lost.

Basically, it's safe if only soft-dirty is allowed to modify vm_flags
without down_write(). But why is soft-dirty so special?

Should we consider moving protection of some vma fields under per-vma lock
rather use over-loaded mmap_sem?

-- 
 Kirill A. Shutemov