Strange memory usage reporting

Strange memory usage reporting

[Måns Rullgård]

I was a little surprised to see top tell me this:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND           
10642 mru       11   0 23200  81m 2740 S  0.0 37.0   0:00.07 tcvp              

It didn't make sense that RES > VIRT, so I check /proc/pid/*.  Their
contents are below.  Am I missing something?  Note that they are not
consistent with the 'top' line above, since they were copied at a
different time.  The effect is easily reproducible.  It happens every
time I run my music player with using ALSA.

The memory usage summary by top, also doesn't agree:

Mem:    224140k total,   219236k used,     4904k free,     7184k buffers
Swap:   524280k total,      224k used,   524056k free,   125976k cached

Subtracting the cached memory and some heavy processes doesn't leave
room for another 80 MB.

/proc/pid/stat:
10642 (tcvp) S 926 10642 926 34816 10642 0 68 0 2556 0 4 3 0 0 11 0 0 0 1162589 25665536 18902 4294967295 134512640 134523016 3221216832 3221215328 1074519393 0 0 0 2 3222307553 0 0 17 0 0 0

/proc/pid/statm:
6266 18902 685 4 0 6262 0

/proc/pid/status:
Name:	tcvp
State:	S (sleeping)
Tgid:	10642
Pid:	10642
PPid:	926
TracerPid:	0
Uid:	51770	51770	51770	51770
Gid:	100	100	100	100
FDSize:	256
Groups:	100 102 
VmSize:	   25064 kB
VmLck:	       0 kB
VmRSS:	   75608 kB
VmData:	   22308 kB
VmStk:	      16 kB
VmExe:	      12 kB
VmLib:	    2492 kB
SigPnd:	0000000000000000
ShdPnd:	0000000000000000
SigBlk:	0000000000000000
SigIgn:	8000000000000000
SigCgt:	0000000380000002
CapInh:	0000000000000000
CapPrm:	0000000000000000
CapEff:	0000000000000000



-- 
Måns Rullgård
mru@users.sf.net

Re: Strange memory usage reporting

[William Lee Irwin III]

On Tue, Aug 26, 2003 at 02:18:54PM +0200, M?ns Rullg?rd wrote:
> I was a little surprised to see top tell me this:
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND           
> 10642 mru       11   0 23200  81m 2740 S  0.0 37.0   0:00.07 tcvp              
> It didn't make sense that RES > VIRT, so I check /proc/pid/*.  Their
> contents are below.  Am I missing something?  Note that they are not
> consistent with the 'top' line above, since they were copied at a
> different time.  The effect is easily reproducible.  It happens every
> time I run my music player with using ALSA.
> The memory usage summary by top, also doesn't agree:

What kernel version?


-- wli

Re: Strange memory usage reporting

[Måns Rullgård]

William Lee Irwin III <wli@holomorphy.com> writes:

> On Tue, Aug 26, 2003 at 02:18:54PM +0200, M?ns Rullg?rd wrote:
>> I was a little surprised to see top tell me this:
>>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND           
>> 10642 mru       11   0 23200  81m 2740 S  0.0 37.0   0:00.07 tcvp              
>> It didn't make sense that RES > VIRT, so I check /proc/pid/*.  Their
>> contents are below.  Am I missing something?  Note that they are not
>> consistent with the 'top' line above, since they were copied at a
>> different time.  The effect is easily reproducible.  It happens every
>> time I run my music player with using ALSA.
>> The memory usage summary by top, also doesn't agree:
>
> What kernel version?

Sorry, I forgot that.  It's 2.6.0-test4 with Nick Piggins' v7
scheduler patch.  The machine I'm running on is a Pentium 4 based
laptop.

-- 
Måns Rullgård
mru@users.sf.net

Re: Strange memory usage reporting

[Jaroslav Kysela]

On Tue, 26 Aug 2003, [iso-8859-1] Mĺns Rullgĺrd wrote:

> I was a little surprised to see top tell me this:
> 
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND           
> 10642 mru       11   0 23200  81m 2740 S  0.0 37.0   0:00.07 tcvp              
> 
> It didn't make sense that RES > VIRT, so I check /proc/pid/*.  Their
> contents are below.  Am I missing something?  Note that they are not
> consistent with the 'top' line above, since they were copied at a
> different time.  The effect is easily reproducible.  It happens every
> time I run my music player with using ALSA.

I have exactly same behaviour with 2.4.21 kernel. It seems that VmRSS
grows with the mmap2 syscalls although appropriate munmap is called. I'm
investigating a possible problem with the memory accounting.

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs

Re: Strange memory usage reporting

[Jaroslav Kysela]

On Tue, 26 Aug 2003, Jaroslav Kysela wrote:

> On Tue, 26 Aug 2003, [iso-8859-1] Mĺns Rullgĺrd wrote:
> 
> > I was a little surprised to see top tell me this:
> > 
> >   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND           
> > 10642 mru       11   0 23200  81m 2740 S  0.0 37.0   0:00.07 tcvp              
> > 
> > It didn't make sense that RES > VIRT, so I check /proc/pid/*.  Their
> > contents are below.  Am I missing something?  Note that they are not
> > consistent with the 'top' line above, since they were copied at a
> > different time.  The effect is easily reproducible.  It happens every
> > time I run my music player with using ALSA.
> 
> I have exactly same behaviour with 2.4.21 kernel. It seems that VmRSS
> grows with the mmap2 syscalls although appropriate munmap is called. I'm
> investigating a possible problem with the memory accounting.

Yes, it seems so. The do_no_page() function in mm/memory.c does accounting 
for reserved pages (++mm->rss), but in zap_pte_range() there is a check 
preventing increase the count of freed pages.

Here is a patch for VM gurus to review (for 2.4 kernel, but it should 
apply to 2.6 as well):

===== mm/memory.c 1.57 vs edited =====
--- 1.57/mm/memory.c	Fri Jun 13 18:26:23 2003
+++ edited/mm/memory.c	Tue Aug 26 15:33:28 2003
@@ -1306,7 +1306,8 @@
 	 */
 	/* Only go through if we didn't race with anybody else... */
 	if (pte_none(*page_table)) {
-		++mm->rss;
+		if (!PageReserved(new_page))
+			++mm->rss;
 		flush_page_to_ram(new_page);
 		flush_icache_page(vma, new_page);
 		entry = mk_pte(new_page, vma->vm_page_prot);


						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs

Re: Strange memory usage reporting

[Hugh Dickins]

On Tue, 26 Aug 2003, Jaroslav Kysela wrote:
> 
> Yes, it seems so. The do_no_page() function in mm/memory.c does accounting 
> for reserved pages (++mm->rss), but in zap_pte_range() there is a check 
> preventing increase the count of freed pages.
> 
> Here is a patch for VM gurus to review (for 2.4 kernel, but it should 
> apply to 2.6 as well):
> 
> ===== mm/memory.c 1.57 vs edited =====
> --- 1.57/mm/memory.c	Fri Jun 13 18:26:23 2003
> +++ edited/mm/memory.c	Tue Aug 26 15:33:28 2003
> @@ -1306,7 +1306,8 @@
>  	 */
>  	/* Only go through if we didn't race with anybody else... */
>  	if (pte_none(*page_table)) {
> -		++mm->rss;
> +		if (!PageReserved(new_page))
> +			++mm->rss;
>  		flush_page_to_ram(new_page);
>  		flush_icache_page(vma, new_page);
>  		entry = mk_pte(new_page, vma->vm_page_prot);

You're right (but please rediff against 2.4.22 when you send Marcelo).

You may wonder how this has taken so long to show up: because usually
drivers which mmap Reserved pages use remap_page_range on them,
and so never fault to do_no_page.

Which is the driver involved?  Though it's not wrong to give do_no_page
a Reserved page, beware of the the page->count accounting: while it's
Reserved, get_page or page_cache_get raises the count, but put_page
or page_cache_release does not decrement it - very easy to end up
with the page never freed.

Hugh

Re: Strange memory usage reporting

[Ingo Oeser]

Hi,

On Tue, Aug 26, 2003 at 06:03:14PM +0100, Hugh Dickins wrote:
> Which is the driver involved?  Though it's not wrong to give do_no_page
> a Reserved page, beware of the the page->count accounting: while it's
> Reserved, get_page or page_cache_get raises the count, but put_page
> or page_cache_release does not decrement it - very easy to end up
> with the page never freed.

Why is this so asymetric? I would understand ignoring these pages
in the freeing logic, but why exclude them also from refcounting?

Regards

Ingo Oeser

Re: Strange memory usage reporting

[Hugh Dickins]

On Wed, 27 Aug 2003, Ingo Oeser wrote:
> On Tue, Aug 26, 2003 at 06:03:14PM +0100, Hugh Dickins wrote:
> > Which is the driver involved?  Though it's not wrong to give do_no_page
> > a Reserved page, beware of the the page->count accounting: while it's
> > Reserved, get_page or page_cache_get raises the count, but put_page
> > or page_cache_release does not decrement it - very easy to end up
> > with the page never freed.
> 
> Why is this so asymetric? I would understand ignoring these pages
> in the freeing logic, but why exclude them also from refcounting?

I don't think there's a _good_ reason, it just evolved that way.

The real answer is to get rid of PageReserved completely, which
I'll embark on again in 2.7 (I did start a couple of times in 2.5,
but each time it was too late).

There was a halfway-house suggestion in 2.5 about three months ago,
inspired (as usual) by Reserved page problems in AIO's get_user_pages,
to do as you suggest: submit them to normal refcounting.  I don't
know what became of that, I didn't have much time to get involved.

Hugh