linux-kernel - put_page on transparent huge page leaks?

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Message-ID: <ff08f2ccbcd50d2d353392a81defe619@jcornwall.me>
Date:	Fri, 21 Feb 2014 11:23:39 -0600
From:	Jay Cornwall <jay@...rnwall.me>
To:	linux-kernel@...r.kernel.org
Subject: put_page on transparent huge page leaks?

Hi,

I'm tracking a possible memory leak in iommu/amd. The driver uses this 
logic to fault a page in response to a PRI from a device:

npages = get_user_pages(fault->state->task, fault->state->mm,
                         fault->address, 1, write, 0, &page, NULL);

if (npages == 1)
         put_page(page);
else
         ...

This works correctly when get_user_pages returns a 4KB page. When 
transparent huge pages are enabled any 2MB page returned by this call 
appears to leak on process exit. The non-cached memory usage stays 
elevated by the set of faulted 2MB pages. This behavior is not observed 
when the exception handler demand faults 2MB pages.

I notice there is a difference in reference count between the 4KB/2MB 
paths.

get_user_pages (4KB): page_count()=3, page_mapcount()=1
put_page       (4KB): page_count()=2, page_mapcount()=1

get_user_pages (2MB): page_count()=3, page_mapcount()=1
put_page       (2MB): page_count()=3, page_mapcount()=0

I'm concerned that the driver appears to be holding a reference count 
after put_page(). Am I interpreting this observation correctly?
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