__get_free_pages page count increment

__get_free_pages page count increment

[omar ramirez]

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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Re: __get_free_pages page count increment

[Alexey Korolev]

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
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
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Re: __get_free_pages page count increment

[omar ramirez]

Hi,

On Tue, Jul 14, 2009 at 9:30 PM, Alexey Korolev<akorolex@gmail.com> wrote:
> 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 for the quick reply it really helped a lot!

-omar

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