Re: Inconsistency (bug) of vm_insert_page with high order allocations

[Mel Gorman <mel@csn.ul.ie>]

On Sat, May 30, 2009 at 05:27:15PM +1200, Alexey Korolev wrote:
> Hi,
> >> To allocate memory I use standard function alloc_apges(gfp_mask,
> >> order) which asks buddy allocator to give a chunk of memory of given
> >> "order".
> >> Allocator returns page and also sets page count to 1 but for page of
> >> high order. I.e. pages 2,3 etc inside high order allocation will have
> >> page->_count==0.
> >> If I try to mmap allocated area to user space vm_insert_page will
> >> return error as pages 2,3, etc are not refcounted.
> >>
> >
> > page = alloc_pages(high_order);
> > split_page(page, high_order);
> >
> > That will fix up the ref-counting of each of the individual pages. You are
> > then responsible for freeing them individually. As you are inserting these
> > into userspace, I suspect that's ok.
> 
> It seems it is the only way I have now. It is not so elegant - but should work.
> Thanks for good advise.
> 
> BTW: Just out of curiosity what limits mapping high ordered pages into
> user space. I tried to find any except the check in vm_insert but
> failed. Is this checks caused by possible swapping?
> 

Nothing limits it as such other than it's usually not required. There is
nothing really that special about high-order pages other than they are
physically contiguous. The expectation is normally that userspace does
not care about physical contiguity.

There is expected to be a 1 to 1 mapping of PTE to ref-counted pages so that
they get freed at the right times so it's not just about swapping.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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