linux-kernel - Re: [Help] : RSS/PSS showing 0 during smaps for Xorg

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Date:	Mon, 23 Jan 2012 11:31:55 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	PINTU KUMAR <pintu_agarwal@...oo.com>
cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [Help] : RSS/PSS showing 0 during smaps for Xorg

On Mon, 23 Jan 2012, PINTU KUMAR wrote:
> Dear All,
> 
> I am facing one problem for one of my kernel module for our linux mobile with kernel2.6.36.
> 
> When I do cat /proc/<Xorg pid>/smaps | grep -A 11 /dev/ump , to track information for my ump module,
> we always get Rss/Pss as 0 kB as shown below:
> cat /proc/1731/smaps | grep -A 11 /dev/ump
> 414db000-415ff000 rw-s 00015000 00:12 6803       /dev/ump
> Size:               1168 kB
> Rss:                   0 kB
> Pss:                   0 kB
> track_rss_value = 0, iswalkcalled = 1, smap_pte_range_called = 1, swap_pte = 0, not_pte_present = 0, not_normal_page = 1
> isspecial = 0, not_special = 1, isMixedMap = 0, pfnpages_null = 0, pfnoff_flag = 0, not_cow_mapping = 1, normal_page_end = 0
>  
> After tracing down the problem, I found out that during "show_smaps" in fs/proc/task_mmu.c and during call to smaps_pte_range the vm_normal_page() is always returning NULL for our /dev/ump driver.
> (smaps_pte_range() is the place where Rss/Pss information is populated)
> Thus mss->resident (Rss value) is never getting incremented.  
>  
> To trace the problem I added few flags during show_smaps & vm_normal_page() as shown above. The value of 1 indicates that the condition is executed.
> Thus "normal_page_end" indicates that the "vm_normal_page" has never ended successfully and always returns from 
> "!is_cow_mapping()".
>  
> So, I wanted to know the main cause for vm_normal_page() always returning NULL page for our ump driver. 
> What is that I am missing in my driver ?
>  
> Can anyone please let me know what could be the problem in our driver.

This not evidence of any problem in your driver.

vm_normal_page() returns NULL because the pages mapped by your driver
are not normal faultable and reclaimable pages, but an area of physical
memory mapped in by remap_pfn_range(), which sets the VM_PFNMAP flag.

The mm subsystem does not count such pages towards rss (or pss),
hence your 0s.

Hugh
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