Re: Hugetlb: Shared memory race

[William Lee Irwin III <wli@holomorphy.com>]

On Tue, Jan 10, 2006 at 01:22:31PM -0600, Adam Litke wrote:
> I have discovered a race caused by the interaction of demand faulting
> with the hugetlb overcommit accounting patch.  Attached is a workaround
> for the problem.  Can anyone suggest a better approach to solving the
> race I'll describe below?  If not, would the attached workaround be
> acceptable?
> The race occurs when multiple threads shmat a hugetlb area and begin
> faulting in it's pages.  During a hugetlb fault, hugetlb_no_page checks
> for the page in the page cache.  If not found, it allocates (and zeroes)
> a new page and tries to add it to the page cache.  If this fails, the
> huge page is freed and we retry the page cache lookup (assuming someone
> else beat us to the add_to_page_cache call).
> The above works fine, but due to the large window (while zeroing the
> huge page) it is possible that many threads could be "borrowing" pages
> only to return them later.  This causes free_hugetlb_pages to be lower
> than the logical number of free pages and some threads trying to shmat
> can falsely fail the accounting check.
> The workaround disables the accounting check that happens at shmat time.
> It was already done at shmget time (which is the normal semantics
> anyway).

So that's where the ->i_blocks bit came from. This is too hacky for me.
Disabling the check raises the spectre of failures when there shouldn't
be. I'd rather have a more invasive fix than a workaround, however tiny.


-- wli

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