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

From: Hugh Dickins <hughd@google.com>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	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: Wed, 27 Aug 2014 16:12:43 -0700 (PDT)	[thread overview]
Message-ID: <alpine.LSU.2.11.1408271512090.7961@eggly.anvils> (raw)
In-Reply-To: <20140826155351.GC8952@moon>

On Tue, 26 Aug 2014, Cyrill Gorcunov wrote:
> On Tue, Aug 26, 2014 at 06:43:55PM +0300, Kirill A. Shutemov wrote:
> > On Tue, Aug 26, 2014 at 07:18:13PM +0400, Cyrill Gorcunov wrote:
> > > > Basically, it's safe if only soft-dirty is allowed to modify vm_flags
> > > > without down_write(). But why is soft-dirty so special?
> > > 
> > > because how we use this bit, i mean in normal workload this bit won't
> > > be used intensively i think so it's not widespread in kernel code
> > 
> > Weak argument to me.

Yes.  However rarely it's modified, we don't want any chance of it
corrupting another flag.

VM_SOFTDIRTY is special in the sense that it's maintained in a very
different way from the other VM_flags.  If we had a little alignment
padding space somewhere in struct vm_area_struct, I think I'd jump at
Kirill's suggestion to move it out of vm_flags and into a new field:
that would remove some other special casing, like the vma merge issue.

But I don't think we have such padding space, and we'd prefer not to
bloat struct vm_area_struct for it; so maybe it should stay for now.
Besides, with Peter's patch, we're also talking about the locking on
modifications to vm_page_prot, aren't we?

> > 
> > What about walk through vmas twice: first with down_write() to modify
> > vm_flags and vm_page_prot, then downgrade_write() and do
> > walk_page_range() on every vma?
> 
> I still it's undeeded,

Yes, so long as nothing else is doing the same.
No bug yet, that we can see, but a bug in waiting.

> but for sure using write-lock/downgrade won't hurt,
> so no argues from my side.

Yes, Kirill's two-stage suggestion seems the best:

down_write
quickly scan vmas clearing VM_SOFT_DIRTY and updating vm_page_prot
downgrade_write (or up_write, down_read?)
slowly walk page tables write protecting and clearing soft-dirty on ptes
up_read

But please don't mistake me for someone who has a good grasp of
soft-dirty: I don't.

Hugh