Re: [PATCH 2/3] Reserve huge pages for reliable MAP_PRIVATEhugetlbfs mappings until fork()

[Adam Litke]

On Tue, 2008-05-27 at 19:51 +0100, Mel Gorman wrote:
> This patch reserves huge pages at mmap() time for MAP_PRIVATE mappings in a
> similar manner to the reservations taken for MAP_SHARED mappings. The reserve count is
> accounted both globally and on a per-VMA basis for private mappings. This
> guarantees that a process that successfully calls mmap() will successfully
> fault all pages in the future unless fork() is called.
> 
> The characteristics of private mappings of hugetlbfs files behaviour after
> this patch are;
> 
> 1. The process calling mmap() is guaranteed to succeed all future faults until
>    it forks().
> 2. On fork(), the parent may die due to SIGKILL on writes to the private
>    mapping if enough pages are not available for the COW. For reasonably
>    reliable behaviour in the face of a small huge page pool, children of
>    hugepage-aware processes should not reference the mappings; such as
>    might occur when fork()ing to exec().
> 3. On fork(), the child VMAs inherit no reserves. Reads on pages already
>    faulted by the parent will succeed. Successful writes will depend on enough
>    huge pages being free in the pool.
> 4. Quotas of the hugetlbfs mount are checked at reserve time for the mapper
>    and at fault time otherwise.
> 
> Before this patch, all reads or writes in the child potentially needs page
> allocations that can later lead to the death of the parent. This applies
> to reads and writes of uninstantiated pages as well as COW. After the
> patch it is only a write to an instantiated page that causes problems.
> 
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>

Acked-by: Adam Litke <agl@xxxxxxxxxx>

-- 
Adam Litke - (agl at us.ibm.com)
IBM Linux Technology Center

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