On 27.02.2008 [23:20:35 -0800], Nishanth Aravamudan wrote:
> On 28.02.2008 [13:55:58 +1100], David Gibson wrote:
> > On Wed, Feb 27, 2008 at 06:34:37PM -0800, Nishanth Aravamudan wrote:
> > > On 28.02.2008 [13:28:47 +1100], David Gibson wrote:
> > > > On Wed, Feb 27, 2008 at 06:17:02PM -0800, Nishanth Aravamudan wrote:
> > [snip]
> > > > > Hrm, I guess I hadn't thought about it that way. I really am curious
> > > > > about these semantics. We already have cases where a small page [heap]
> > > > > exists in a processes address space, say at 0x10000000 for a 32-bit
> > > > > process and we start the hugepage heap in the morecore constructor at
> > > > > 0x20000000. If glibc thought it could use that entire area, but we
> > > > > didn't actually allocate any pages for it to use there, does it just
> > > > > take a bunch of page faults?
> > > >
> > > > We really have such cases? I wouldn't have expected that to work.
> > >
> > > We definitely do. It's been a source of confusion for the testers, but
> > > it does seem to work. And given the code I pasted earlier in the thread
> > > (from morecore.c), it's clearly been coded to be a non-failure case if
> > > the first mmap() is moved (mapsize == 0), but if we've already allocated
> > > any of the heap, we fail.
> >
> > Hrm. Have we checked in these cases that malloc() or (glibc's
> > default) morecore is actually being called before our version. I'm
> > wondering if it might just be that the tail end of the BSS is being
> > labelled "[heap]" in /proc/pid/maps, but we don't actually have a heap
> > from the glibc allocator's point of view.
>
> Hrm, I suppose that could be true. maps sees the brk() and says [heap]
> must be there, but it might not be.
>
> How would we (I) figure out if malloc() or morecore has been called
> before us?
Actually thinking about it, I suppose strace() could help a little?
Looking at the kernel, the following snippet from fs/proc/task_mmu.c is
the relevant bit:
if (vma->vm_start <= mm->start_brk &&
vma->vm_end >= mm->brk) {
name = "[heap]";
} else if (vma->vm_start <= mm->start_stack &&
vma->vm_end >= mm->start_stack) {
name = "[stack]";
}
So if the VMA we're looking at starts at or before the original brk()
and ends at or after the current brk(), it's considered the heap.
How would that be the case if it's the tail-end of the heap, since we
are not setting the brk()?
Thanks,
Nish
--
Nishanth Aravamudan <[EMAIL PROTECTED]>
IBM Linux Technology Center
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