Re: [PATCH 1/5] mm, shmem: Add shmem resident memory accounting

[Jerome Marchand]

On 08/01/2014 07:01 AM, Hugh Dickins wrote:
> On Tue, 22 Jul 2014, Jerome Marchand wrote:
> 
>> Currently looking at /proc/<pid>/status or statm, there is no way to
>> distinguish shmem pages from pages mapped to a regular file (shmem
>> pages are mapped to /dev/zero), even though their implication in
>> actual memory use is quite different.
>> This patch adds MM_SHMEMPAGES counter to mm_rss_stat. It keeps track of
>> resident shmem memory size. Its value is exposed in the new VmShm line
>> of /proc/<pid>/status.
> 
> I like adding this info to /proc/<pid>/status - thank you -
> but I think you can make the patch much better in a couple of ways.
> 
>>
>> Signed-off-by: Jerome Marchand <jmarchan@xxxxxxxxxx>
>> ---
>>  Documentation/filesystems/proc.txt |  2 ++
>>  arch/s390/mm/pgtable.c             |  2 +-
>>  fs/proc/task_mmu.c                 |  9 ++++++---
>>  include/linux/mm.h                 |  7 +++++++
>>  include/linux/mm_types.h           |  7 ++++---
>>  kernel/events/uprobes.c            |  2 +-
>>  mm/filemap_xip.c                   |  2 +-
>>  mm/memory.c                        | 37 +++++++++++++++++++++++++++++++------
>>  mm/rmap.c                          |  8 ++++----
>>  9 files changed, 57 insertions(+), 19 deletions(-)
>>
>> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
>> index ddc531a..1c49957 100644
>> --- a/Documentation/filesystems/proc.txt
>> +++ b/Documentation/filesystems/proc.txt
>> @@ -171,6 +171,7 @@ read the file /proc/PID/status:
>>    VmLib:      1412 kB
>>    VmPTE:        20 kb
>>    VmSwap:        0 kB
>> +  VmShm:         0 kB
>>    Threads:        1
>>    SigQ:   0/28578
>>    SigPnd: 0000000000000000
>> @@ -228,6 +229,7 @@ Table 1-2: Contents of the status files (as of 2.6.30-rc7)
>>   VmLib                       size of shared library code
>>   VmPTE                       size of page table entries
>>   VmSwap                      size of swap usage (the number of referred swapents)
>> + VmShm	                      size of resident shmem memory
> 
> Needs to say that includes mappings of tmpfs, and needs to say that
> it's a subset of VmRSS.  Better placed immediately after VmRSS...
> 
> ...but now that I look through what's in /proc/<pid>/status, it appears
> that we have to defer to /proc/<pid>/statm to see MM_FILEPAGES (third
> field) and MM_ANONPAGES (subtract third field from second field).
> 
> That's not a very friendly interface.  If you're going to help by
> exposing MM_SHMPAGES separately, please help even more by exposing
> VmFile and VmAnon here in /proc/<pid>/status too.
> 

Good point.

> VmRSS, VmAnon, VmShm, VmFile?  I'm not sure what's the best order:
> here I'm thinking that anon comes before file in /proc/meminfo, and
> shm should be halfway between anon and file.  You may have another idea.
> 
> And of course the VmFile count here should exclude VmShm: I think it
> will work out least confusingly if you account MM_FILEPAGES separately
> from MM_SHMPAGES, but add them together where needed e.g. for statm.

I chose not to change MM_FILEPAGES to avoid to break anything, but it
might indeed look better not to have MM_SHMPAGES included in
MM_FILEPAGES. I'll look into it.

> 
>>   Threads                     number of threads
>>   SigQ                        number of signals queued/max. number for queue
>>   SigPnd                      bitmap of pending signals for the thread
>> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
>> index 37b8241..9fe31b0 100644
>> --- a/arch/s390/mm/pgtable.c
>> +++ b/arch/s390/mm/pgtable.c
>> @@ -612,7 +612,7 @@ static void gmap_zap_swap_entry(swp_entry_t entry, struct mm_struct *mm)
>>  		if (PageAnon(page))
>>  			dec_mm_counter(mm, MM_ANONPAGES);
>>  		else
>> -			dec_mm_counter(mm, MM_FILEPAGES);
>> +			dec_mm_file_counters(mm, page);
>>  	}
> 
> That is a recurring pattern: please try putting
> 
> static inline int mm_counter(struct page *page)
> {
> 	if (PageAnon(page))
> 		return MM_ANONPAGES;
> 	if (PageSwapBacked(page))
> 		return MM_SHMPAGES;
> 	return MM_FILEPAGES;
> }
> 
> in include/linux/mm.h.
> 
> Then dec_mm_counter(mm, mm_counter(page)) here, and wherever you can,
> use mm_counter(page) to simplify the code throughout.
> 
> I say "try" because I think factoring out mm_counter() will simplify
> the most code, given the profusion of different accessors, particularly
> in mm/memory.c.  But I'm not sure how much bloat having it as an inline
> function will add, versus how much overhead it would add if not inline.

I'll look into that.

Jerome

> 
> Hugh
>

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