linux-kernel - Re: [PATCH] make MADV

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Date:	Mon, 16 Apr 2007 12:30:57 -0400
From:	Jakub Jelinek <jakub@...hat.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Rik van Riel <riel@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Eric Dumazet <dada1@...mosbay.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Ulrich Drepper <drepper@...hat.com>
Subject: Re: [PATCH] make MADV_FREE lazily free memory

On Mon, Apr 16, 2007 at 11:10:39AM -0500, Anton Blanchard wrote:
> > Making the pte clean also needs to clear the hardware writable
> > bit on architectures where we do pte dirtying in software.
> > 
> > If we don't, we would have corruption problems all over the VM,
> > for example in the code around pte_clean_one :)
> > 
> > >But as Linus recently said, even hardware handled faults still
> > >take expensive microarchitectural traps.
> > 
> > Nowhere near as expensive as a full page fault, though...
> 
> Unfortunately it will be expensive on architectures that have software
> referenced and changed. It would be great if we could just leave them
> dirty in the pagetables and transition between a clean and dirty state
> via madvise calls, but thats just wishful thinking on my part :)

That would mean an additional syscall.  Furthermore, if you allocate a big
chunk of memory, dirty it, then free (with madvise (MADV_FREE)) it and soon
allocate the same size of memory again, it is better to start that with
non-dirty memory, it might be that this time you e.g. don't modify a big
part of the chunk.  If all that memory was kept dirty all the time and
just marked/unmarked for lazy reuse with MADV_FREE/MADV_UNDO_FREE, all that
memory would need to be saved to disk when paging out as it was marked
dirty, while with current Rik's MADV_FREE that will happen only for pages
that were actually dirtied after the last malloc.

	Jakub
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