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SubjectRe: bisected boot regression post 2.6.25-rc3.. please revert
On 3/1/08, Arjan van de Ven <arjan@linux.intel.com> wrote:
> 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>

Hrm, not that I necessarily have the right to ask for this, but
shouldn't hugepage related patches at least be cc'd to linux-mm? it
seems this went via LKML to the x86 tree? There are a few of us that
work on hugepages that would have seen this there, but it got lost in
the noise (for me) on LKML.

Thanks,
Nish


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