linux-kernel - Re: [RESEND PATCH V2 1/3] Add mmap flag to request pages are locked after page fault

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Message-ID: <20150624094742.GD32756@dhcp22.suse.cz>
Date:	Wed, 24 Jun 2015 11:47:42 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Eric B Munson <emunson@...mai.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mips@...ux-mips.org, linux-parisc@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
	linux-xtensa@...ux-xtensa.org, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [RESEND PATCH V2 1/3] Add mmap flag to request pages are locked
 after page fault

On Tue 23-06-15 14:45:17, Vlastimil Babka wrote:
> On 06/22/2015 04:18 PM, Eric B Munson wrote:
> >On Mon, 22 Jun 2015, Michal Hocko wrote:
> >
> >>On Fri 19-06-15 12:43:33, Eric B Munson wrote:
[...]
> >>>My thought on detecting was that someone might want to know if they had
> >>>a VMA that was VM_LOCKED but had not been made present becuase of a
> >>>failure in mmap.  We don't have a way today, but adding VM_LOCKONFAULT
> >>>is at least explicit about what is happening which would make detecting
> >>>the VM_LOCKED but not present state easier.
> >>
> >>One could use /proc/<pid>/pagemap to query the residency.
> 
> I think that's all too much complex scenario for a little gain. If someone
> knows that mmap(MAP_LOCKED|MAP_POPULATE) is not perfect, he should either
> mlock() separately from mmap(), or fault the range manually with a for loop.
> Why try to detect if the corner case was hit?

No idea. I have just offered a way to do that. I do not think it is
anyhow useful but who knows... I do agree that the mlock should be used
for the full mlock semantic.

> >>>This assumes that
> >>>MAP_FAULTPOPULATE does not translate to a VMA flag, but it sounds like
> >>>it would have to.
> >>
> >>Yes, it would have to have a VM flag for the vma.
> 
> So with your approach, VM_LOCKED flag is enough, right? The new MAP_ /
> MLOCK_ flags just cause setting VM_LOCKED to not fault the whole vma, but
> otherwise nothing changes.

VM_FAULTPOPULATE would have to be sticky to prevent from other
speculative poppulation of the mapping. I mean, is it OK to have a new
mlock semantic (on fault) which might still populate&lock memory which
hasn't been faulted directly? Who knows what kind of speculative things
we will do in the future and then find out that the semantic of
lock-on-fault is not usable anymore.

[...]

-- 
Michal Hocko
SUSE Labs
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