On 5/23/19 3:37 PM, Ira Weiny wrote:
[...]
> I've dug in further and I see now that release_pages() implements (almost the
> same thing, see below) as put_page().
>
> However, I think we need to be careful here because put_page_testzero() calls
>
> page_ref_dec_and_test(page);
>
> ... and after your changes it will need to call ...
>
> page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
>
> ... on a GUP page:
>
> So how do you propose calling release_pages() from within put_user_pages()?
> Or
> were you thinking this would be temporary?
I was thinking of it as a temporary measure, only up until, but not including
the
point where put_user_pages() becomes active. That is, the point when
put_user_pages
starts decrementing GUP_PIN_COUNTING_BIAS, instead of just forwarding to
put_page().
(For other readers, that's this patch:
"mm/gup: debug tracking of get_user_pages() references"
...in https://github.com/johnhubbard/linux/tree/gup_dma_core )
>
> That said, there are 2 differences I see between release_pages() and
> put_page()
>
> 1) release_pages() will only work for a MEMORY_DEVICE_PUBLIC page and not all
> devmem pages...
> I think this is a bug, patch to follow shortly.
>
> 2) release_pages() calls __ClearPageActive() while put_page() does not
>
> I have no idea if the second difference is a bug or not. But it smells of
> one...
>
> It would be nice to know if the open coding of put_page is really a
> performance
> benefit or not. It seems like an attempt to optimize the taking of the page
> data lock.
>
> Does anyone have any information about the performance advantage here?
>
> Given the changes above it seems like it would be a benefit to merge the 2
> call
> paths more closely to make sure we do the right thing.
>
Yes, it does. Maybe best to not do the temporary measure, then, while this stuff
gets improved. I'll look at your other patch...
thanks,
--
John Hubbard
NVIDIA
