On Wed, 2005-10-19 at 16:53 -0700, Rohit Seth wrote:
> On Wed, 2005-10-19 at 21:28 +0100, Hugh Dickins wrote:
> > On Wed, 19 Oct 2005, Andrew Morton wrote:
> > > Hugh Dickins <[email protected]> wrote:
> > > >
> > > > I was forgetting that extending ftruncate wasn't supported in 2.6.14 and
> > > > earlier, yes. But I'm afraid the above scenario can still happen there:
> > > > extending is done, not by ftruncate, but by (somewhere else) mmapping the
> > > > larger size. So your fix may still cause a tight infinite fault loop.
> > >
> > > Will it? Whenever we mmap a hugetlbfs file we prepopulate the entire vma
> > > with hugepages. So I don't think there's ever any part of an address space
> > > which ia a) inside a hugepage vma and b) doesn't have a hugepage backing
> > > it.
> >
> > The new vma, sure, will be fully populated. But the old vma, in this
> > or some other process, which was created before the hugetlbfs file was
> > truncated down, will be left with a hole at the end.
> >
>
> Excellent catch. This broken truncation thing....
>
> And I don't know what the right solution should be for this scenario at
> this point for 2.6.14....may be to actually look at the HUGEPTE
> corresponding to the hugetlb faulting address or don't allow mmaps to
> grow the hugetlb file bigger (except the first mmap). I understand that
> both of them don't sound too good...
>
> Any suggestions.
>
I would like to keep this patch. This at least takes care of bad things
happening behind application's back by OS/HW. If the scenario that you
mentioned happens then the application is knowingly doing unsupported
things (at this point truncate is a broken operation on hugetlb). The
application can be killed in this case.
...trying to avoid any heavy changes at this time for 2.6.14
-rohit
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