linux-kernel - Re: [PATCH v3 1/3] mm: introduce fincore()

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Date:	Mon, 07 Jul 2014 13:43:31 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Johannes Weiner <hannes@...xchg.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	David Miller <davem@...emloft.net>,
	Andres Freund <andres@...quadrant.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Naoya Horiguchi <nao.horiguchi@...il.com>,
	Kees Cook <kees@...flux.net>
Subject: Re: [PATCH v3 1/3] mm: introduce fincore()

On 07/07/2014 01:21 PM, Naoya Horiguchi wrote:
> On Mon, Jul 07, 2014 at 12:01:41PM -0700, Dave Hansen wrote:
>> But, is this trying to do too many things at once?  Do we have solid use
>> cases spelled out for each of these modes?  Have we thought out how they
>> will be used in practice?
> 
> tools/vm/page-types.c will be an in-kernel user after this base code is
> accepted. The idea of doing fincore() thing comes up during the discussion
> with Konstantin over file cache mode of this tool.
> pfn and page flag are needed there, so I think it's one clear usecase.

I'm going to take that as a no. :)

The whole FINCORE_PGOFF vs. FINCORE_BMAP issue is something that will
come up in practice.  We just don't have the interfaces for an end user
to pick which one they want to use.
>> Is it really right to say this is going to be 8 bytes?  Would we want it
>> to share types with something else, like be an loff_t?
> 
> Could you elaborate it more?

We specify file offsets in other system calls, like the lseek family.  I
was just thinking that this type should match up with those calls since
they are expressing the same data type with the same ranges and limitations.

>>> + * - FINCORE_PFN:
>>> + *     stores pfn, using 8 bytes.
>>
>> These are all an unprivileged operations from what I can tell.  I know
>> we're going to a lot of trouble to hide kernel addresses from being seen
>> in userspace.  This seems like it would be undesirable for the folks
>> that care about not leaking kernel addresses, especially for
>> unprivileged users.
>>
>> This would essentially tell userspace where in the kernel's address
>> space some user-controlled data will be.
> 
> OK, so this and FINCORE_PAGEFLAGS will be limited for privileged users.

Then I'd just question their usefulness outside of a debugging
environment, especially when you can get at them in other (more
roundabout) ways in a debugging environment.

This is really looking to me like two system calls.  The bitmap-based
one, and another more extensible one.  I don't think there's any harm in
having two system calls, especially when they're trying to glue together
two disparate interfaces.

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