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

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

On Thu, May 28, 2009 at 05:07:01PM +1200, Alexey Korolev wrote:
> Hi,
> I have the following issue. I need to allocate a big chunk of
> contiguous memory and then transfer it to user mode applications to
> let them operate with given buffers.
> 
> 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.

> The issue could be workaround if to set-up refcount to 1 manually for
> each page. But this workaround is not very good, because page refcount
> is used inside mm subsystem only.
> 

And you would have reimplemented split_page().

> While searching a driver with the similar solutions in kernel tree it
> was found a driver which suffers from exactly the same
> problem("poch"). So it is not single problem.
> 
> What you could suggest to workaround the problem except hacks with page count?
> May be it makes sence to introduce wm_insert_pages function?
> 
> In this case users would have the following picture:
> zero order page: alloc_page <-> vm_instert_page
> non zero order  : alloc_pages(..., order) <-> vm_instert_pages(...., order)
> 

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

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