Adam,
I just commented out printfs, but still program gets a seg fault. I have a
tight loop after "munmap", so ideally program should never come out, but for me
program gets a SIGSEGV after first munmap....
thanks,
Mehul.
--- On Mon, 1/5/09, Adam Litke <a...@us.ibm.com> wrote:
From: Adam Litke <a...@us.ibm.com>
Subject: Re: [Libhugetlbfs-devel] Lib hugetlbfs
To: "Nishanth Aravamudan" <n...@us.ibm.com>
Cc: "mehul vora" <mehu...@yahoo.com>, libhugetlbfs-devel@lists.sourceforge.net
Date: Monday, January 5, 2009, 12:19 PM
On Mon, 2009-01-05 at 09:07 -0800, Nishanth Aravamudan wrote:
> On 05.01.2009 [06:26:55 -0800], mehul vora wrote:
> > Nishanth,
> >
> > I went through "munmap" code and found that core kernel
(arch
> > independent) code removes all "vma" references of the
passed vaddr and
> > then it calls arch dependent hooks. MIPS code removes all pagetable
> > reference of the passed vaddr and flushes tlbs on all cpus. So after
> > munmap returns, if process will try to access the same address again
> > then it will receive a SIGSEGV signal.
>
> We do call it a black hole, as I've said :) It's hairy code.
>
> > I wrote a simple program (not linked with libhugetlbfs - see
> > attachment) to cross check this on MIPS. The moment program does the
> > "munmap" of the code segment it immediately receives a
SIGSEGV. Same
> > program also crashes on x86 with SIGSEGV.
>
> Right, I see that.
>
> > Is this program, at a high level, very similar to libhugetlb init
> > (constructor - munmap/mmap sequence) code ? If yes, then, Is there
> > anything specific libhugetlbfs does before unmapping the code segment
> > apart from copying all segments in to hugepages to make this work ?
>
> A few thoughts: we don't unmap all segments, only those that need to
be
> remapped. Not sure if that matters. The only external reference we
> resolve between unmapping and mapping is mmap(), which is why we have
> that false mmap() call. So, nothing else should access the text segment
> between unmap and map in libhugetlbfs. I know libhugetlbfs works on x86
> :) I'll think about it further. Maybe someone else will have some
ideas.
You are not allowed to call libc functions while segments are unmapped.
Calling printf (for example) will cause a lookup of the symbol stdout
and may trigger dynamic symbol lookups.
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