Re: [PATCH 0/2] pageattr fixes for pmd/pte_present

[Simon Jeons <simon.jeons@gmail.com>]

On Mon, 2012-12-17 at 19:00 +0100, Andrea Arcangeli wrote:
> Hi,
> 
> I got a report for a minor regression introduced by commit
> 027ef6c87853b0a9df53175063028edb4950d476.
> 
> So the problem is, pageattr creates kernel pagetables (pte and pmds)
> that breaks pte_present/pmd_present and the patch above exposed this
> invariant breakage for pmd_present.
> 
> The same problem already existed for the pte and pte_present and it
> was fixed by commit 660a293ea9be709b893d371fbc0328fcca33c33a (if it
> wasn't for that commit, it wouldn't even be a regression). That fix
> avoids the pagefault to use pte_present. I could follow through by
> stopping using pmd_present/pmd_huge too.
> 
> However I think it's more robust to fix pageattr and to clear the
> PSE/GLOBAL bitflags too in addition to the present bitflag. So the
> kernel page fault can keep using the regular
> pte_present/pmd_present/pmd_huge.
> 
> The confusion arises because _PAGE_GLOBAL and _PAGE_PROTNONE are
> sharing the same bit, and in the pmd case we pretend _PAGE_PSE to be
> set only in present pmds (to facilitate split_huge_page final tlb
> flush).

Hi all,

One offline question.

Why free pages should map to kernel virtual address in function
isolate_freepages? If you said that those pages are about to be used as
migration targets, it's splitting a free page but those pages are about
to be used for copying data to, then why kernel need to access these
pages? These target pages are isolated buddy pages and they will be the
migration targets of normal processes' pages, why kernel need to access
them? 

> 
> Andrea Arcangeli (2):
>   Revert "x86, mm: Make spurious_fault check explicitly check the
>     PRESENT bit"
>   pageattr: prevent PSE and GLOABL leftovers to confuse pmd/pte_present
>     and pmd_huge
> 
>  arch/x86/mm/fault.c    |    8 +------
>  arch/x86/mm/pageattr.c |   50 +++++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 48 insertions(+), 10 deletions(-)
> 
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