linux-kernel - Re: Why preallocate pmd in x86 32-bit PAE?

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Message-ID: <473DF2EF.70404@goop.org>
Date:	Fri, 16 Nov 2007 11:43:43 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	William Lee Irwin III <wli@...omorphy.com>,
	Andi Kleen <ak@...e.de>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Why preallocate pmd in x86 32-bit PAE?

Linus Torvalds wrote:
> On Fri, 16 Nov 2007, Jeremy Fitzhardinge wrote:
>   
>> If you happened to do a 1G munmap, it may end up freeing and 
>> reallocating them, but that's going to be very rare.
>>     
>
> I don't think we ever free the pmd's now, do we?
>
> (Except for the *final* free, of course, when we release the whole VM).

Not for 32-bit at the moment, but it does in principle.  munmap ends up
calling free_pgtables, and so ends up calling pmd_free_range.  That will
do a pud_clear to detach the pmd from the pagetable and call
__pmd_free_tlb, which ends up doing tlb_remove_page ->
free_page_and_swap_cache.  32-bit knobbles all this at the moment, but
it looks to me like it wouldn't be hard to make this work if the code is
all common with 64-bit.

    J
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