Re: __get_free_pages page count increment

[Alexey Korolev <akorolex@gmail.com>]

Hi,

About two months ago I faced pretty much the same issue.
Yes it is a proper behaviour. Please see thread
http://marc.info/?l=linux-mm&m=124348722701100&w=2

The best solution for your case  would be involving split_page() function.

Thanks
Alexey

On Wed, Jul 15, 2009 at 12:02 PM, omar ramirez<or.rmz1@gmail.com> wrote:
> Hi,
>
> I have been digging about __get_free_pages function, and wanted to now
> why only the first page reserved with this function increments the
> page count and for the other they are marked as 0.
>
> So here it is what I'm doing, I'm reserving a chunk of pages (using
> __get_free_pages) in a display driver, then I pass that through
> userspace to a dsp driver to decode a video file and return the buffer
> to display.
>
> The buffer is mapped in the dsp, which also follows the get_page
> approach but it goes page-by-page on the buffer, incrementing the page
> count for all of the pages (so now first page count from the buffer
> will be 2 <display, dsp>, but for the rest it will be 1 <dsp>).
>
> The issue comes once those pages are unmapped from the dsp driver,
> because it will do a page_cache_release on all the reserved pages
> (which leave the count as it was before, first page 1 <display> and
> the rest as 0).
>
> This will throw the BUG: bad page state error because the count is
> being marked as 0 for the process using that buffer.
>
> So my question is, is it ok that the page count is NOT incremented for
> all but first page of __get_free_pages?
>
> Thanks in advance,
>
> omar
>
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