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

[Alexey Korolev <akorolex@gmail.com>]

Mel, Kame San,

Thanks a lot for your answers and good advises it is more or less clear why
counting needs to be per page based.
Code which splits pages works fine - no issues.

On Tue, Jun 2, 2009 at 8:38 PM, Mel Gorman<mel@csn.ul.ie> wrote:
> 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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