Re: [PATCH v11 00/25] mm/gup: track dma-pinned pages: FOLL_PIN

John Hubbard
2019-12-20T23:55:11Z

On 12/20/19 10:29 AM, Leon Romanovsky wrote:
...
>> $ ./build.sh
>> $ build/bin/run_tests.py
>>
>> If you get things that far I think Leon can get a reproduction for you
>
> I'm not so optimistic about that.
>

OK, I'm going to proceed for now on the assumption that I've got an overflow
problem that happens when huge pages are pinned. If I can get more information,
great, otherwise it's probably enough.

One thing: for your repro, if you know the huge page size, and the system
page size for that case, that would really help. Also the number of pins per
page, more or less, that you'd expect. Because Jason says that only 2M huge
pages are used...

Because the other possibility is that the refcount really is going negative,
likely due to a mismatched pin/unpin somehow.

If there's not an obvious repro case available, but you do have one (is it easy
to repro, though?), then *if* you have the time, I could point you to a github
branch that reduces GUP_PIN_COUNTING_BIAS by, say, 4x, by applying this:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index bb44c4d2ada7..8526fd03b978 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1077,7 +1077,7 @@ static inline void put_page(struct page *page)
* get_user_pages and page_mkclean and other calls that race to set up page
* table entries.
*/
-#define GUP_PIN_COUNTING_BIAS (1U << 10)
+#define GUP_PIN_COUNTING_BIAS (1U << 8)

void unpin_user_page(struct page *page);
void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,

If that fails to repro, then we would be zeroing in on the root cause.

The branch is here (I just tested it and it seems healthy):

git@github.com:johnhubbard/linux.git pin_user_pages_tracking_v11_with_diags

thanks,
--
John Hubbard
NVIDIA