lkml.org 
[lkml]   [2008]   [May]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] x86: fix PAE pmd_bad bootup warning
On Thu, 8 May 2008, Nishanth Aravamudan wrote:
>
> So, is there any way to either add a is_vm_hugetlb_page(vma) check into
> pagemap_read()? Or can we modify walk_page_range to take the a vma and
> skip the walking if is_vm_hugetlb_page(vma) is set [to avoid
> complications down the road until hugepage walking is fixed]. I guess
> the latter isn't possible for pagemap_read(), since we are just looking
> at arbitrary addresses in the process space?
>
> Dunno, seems quite clear that the bug is in pagemap_read(), not any
> hugepage code, and that the simplest fix is to make pagemap_read() do
> what the other walker-callers do, and skip hugepage regions.

Yes, I'm afraid it needs an is_vm_hugetlb_page(vma) in there somehow:
as you observe, that's what everything else uses to avoid huge issues.

A pmd_huge(*pmd) test is tempting, but it only ever says "yes" on x86:
we've carefully left it undefined what happens to the pgd/pud/pmd/pte
hierarchy in the general arch case, once you're amongst hugepages.

Might follow_huge_addr() be helpful, to avoid the need for a vma?
Perhaps, but my reading is that actually we've never really been
testing that path's success case (because get_user_pages already
skipped is_vm_hugetlb_page), so it might hold further surprises
on one architecture or another.

Many thanks to Hans for persisting, and pointing us to pagemap
to explain this hugepage leak: yes, the pmd_none_or_clear_bad
will be losing it - and corrupting target user address space.

Cc'ed Matt: he may have a view on what he wants his pagewalker
to do with hugepages: I fear it would differ from one usage to
another. Skip over them has to be safest, though not ideal.

Hugh


\
 
 \ /
  Last update: 2008-05-08 21:03    [W:0.073 / U:0.688 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site