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Subjectbisected boot regression post 2.6.25-rc3.. please revert
Hi Linus, Ingo, Hans,

Please revert commit cded932b75ab0a5f9181ee3da34a0a488d1a14fd
( x86: fix pmd_bad and pud_bad to support huge pages )
since it prevents the kernel to finish booting on my (Penryn based)
laptop. The boot stops right after freeing the init memory.
Took a while to bisect (due to it touching page*.h, which forces a
full recompile), but it definitely is caused by this commit...

Please revert.



[arjan@localhost linux.trees.git]$ git-bisect good
cded932b75ab0a5f9181ee3da34a0a488d1a14fd is first bad commit
commit cded932b75ab0a5f9181ee3da34a0a488d1a14fd
Author: Hans Rosenfeld <hans.rosenfeld@amd.com>
Date: Mon Feb 18 18:10:47 2008 +0100

x86: fix pmd_bad and pud_bad to support huge pages

I recently stumbled upon a problem in the support for huge pages. If a
program using huge pages does not explicitly unmap them, they remain
mapped (and therefore, are lost) after the program exits.

I observed that the free huge page count in /proc/meminfo decreased when
running my program, and it did not increase after the program exited.
After running the program a few times, no more huge pages could be
allocated.

The reason for this seems to be that the x86 pmd_bad and pud_bad
consider pmd/pud entries having the PSE bit set invalid. I think there
is nothing wrong with this bit being set, it just indicates that the
lowest level of translation has been reached. This bit has to be (and
is) checked after the basic validity of the entry has been checked, like
in this fragment from follow_page() in mm/memory.c:

if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
goto no_page_table;
if (pmd_huge(*pmd)) {
BUG_ON(flags & FOLL_GET);
page = follow_huge_pmd(mm, address, pmd, flags & FOLL_WRITE);
goto out;
}
Note that this code currently doesn't work as intended if the pmd refers
to a huge page, the pmd_huge() check can not be reached if the page is
huge.

Extending pmd_bad() (and, for future 1GB page support, pud_bad()) to
allow for the PSE bit being set fixes this. For similar reasons,
allowing the NX bit being set is necessary, too. I have seen huge pages
having the NX bit set in their pmd entry, which would cause the same
problem.

Signed-Off-By: Hans Rosenfeld <hans.rosenfeld@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

:040000 040000 1051a11e76304c0fa9229af65cbbd288a8125651
fd91df38defe41df09dde3c0955fb32a4476467a M include


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