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From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@transmeta.com>
Cc: Andrew Morton <akpm@digeo.com>,
	Rusty Russell <rusty@rustcorp.com.au>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [patch] 'sticky pages' support in the VM, futex-2.5.38-C5
Date: Fri, 27 Sep 2002 09:53:40 +0200 (CEST)	[thread overview]
Message-ID: <Pine.LNX.4.44.0209270939330.3174-100000@localhost.localdomain> (raw)
In-Reply-To: <Pine.LNX.4.33.0209261530150.1345-100000@penguin.transmeta.com>


On Thu, 26 Sep 2002, Linus Torvalds wrote:

> Well, we have two situations:
>  - the page is shared. In which case we don't need to put it on any 
>    pinning list, since the very sharedness of the page means that we won't
>    be COW'ing it in this address space.
>  - the page is private. In which case we can (and should) pre-COW it and 
>    make it anonymous at futex time.
> 
> So yeah, it should be doable.

well, 4 fields: ->mapping, ->list.next, ->list.prev, ->index and ->private
is *alot* of space to do something smart in struct page itself.

(hm, dont we have named anonymous mappings? (ie. mmap()-ing of a file via
MAP_PRIVATE?) And if a futex is put there it can be COW-ed, right, while
it's also in the pagecache?)

assuming those 4 fields are available, it should be easy for the futex
code to detect such mappings - the ->mapping field should be a good test,
if it's NULL then it's a COW-able page, if it's non-NULL then not.

then eg. the ->private field could be used as a simple PG_sticky counter.

or, to implement real lazy COW, the ->private field could be used as an
'owner MM' pointer, and if the COW handler sees current->mm == owner_mm,
then the ->list has a queue of pending page_change_struct's, which would
trigger a rehashing of the futexes. Then PG_sticky is cleared.

this would work fine as long as the pin_page code guarantees to never put
a hash on an already COW-ed page. (which can be guaranteed by using the
'writable' flag to get_user_pages())

no additional hashes. Is there any danger in going into this direction?

	Ingo


  reply	other threads:[~2002-09-27  7:39 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2002-09-26 11:30 Ingo Molnar
2002-09-26 15:01 ` Linus Torvalds
2002-09-26 22:09 ` Andrew Morton
2002-09-26 22:32   ` Linus Torvalds
2002-09-27  7:53     ` Ingo Molnar [this message]
2002-09-26 22:45   ` Linus Torvalds
2002-09-26 22:56     ` Linus Torvalds
2002-09-27 11:11     ` Ingo Molnar
2002-10-04 22:47 ` Jamie Lokier
2002-10-04 23:20   ` Linus Torvalds
     [not found] <200209261501.g8QF1pc02251@penguin.transmeta.com>
2002-09-26 15:27 ` Ingo Molnar
2002-09-26 16:48   ` Alan Cox
2002-09-27  8:05 Martin Wirth
2002-09-27  9:27 ` Ingo Molnar

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