linux-kernel - Re: bisected boot regression post 2.6.25-rc3.. please revert

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Date:	Mon, 3 Mar 2008 09:15:22 -0800
From:	"Nish Aravamudan" <nish.aravamudan@...il.com>
To:	"Arjan van de Ven" <arjan@...ux.intel.com>
Cc:	mingo@...e.hu, torvalds@...ux-foundation.org,
	hans.rosenfeld@....com, linux-kernel@...r.kernel.org
Subject: Re: bisected boot regression post 2.6.25-rc3.. please revert

On 3/1/08, Arjan van de Ven <arjan@...ux.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@...alhost linux.trees.git]$ git-bisect good
>  cded932b75ab0a5f9181ee3da34a0a488d1a14fd is first bad commit
>  commit cded932b75ab0a5f9181ee3da34a0a488d1a14fd
>  Author: Hans Rosenfeld <hans.rosenfeld@....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@....com>
>     Signed-off-by: Ingo Molnar <mingo@...e.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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