Re: SysV swapped shared memory calculated incorrectly

[Hugh Dickins <hugh.dickins@tiscali.co.uk>]

On Tue, 4 Aug 2009, Niko Jokinen wrote:
> 
> Tested on 2.6.28 and 2.6.31-rc4
> 
> SysV swapped shared memory is not calculated correctly
> in /proc/<pid>/smaps and also by parsing /proc/<pid>/pagemap.

smaps and pagemap are (reasonably) counting swap entries in the
page tables they're looking at.

But SysV shared memory is dealt with just like mmap of a tmpfs
file: we don't put swap entries into the page tables for that,
just as we don't put sector numbers into the page tables when
unmapping a diskfile page; the use of swapspace by that
filesystem is a lower-level detail not exposed at this level.

Well, we have had to expose "swap backed" near this level in
recent releases.  So it would be possible to recognize the
swap-backed shared vmas, and insert pte_file ptes instead
of pte_none ptes when unmapping pages from them, and adjust
the code which only expects those in nonlinear vmas, and
adjust smaps and pagemap to behave accordingly.

But I admit to having no appetite for any such change, cluttering
the main code just to touch up the anyhow rough picture that smaps
and pagemap are painting.  I much prefer to say that these areas are
backed by files, and it's a lower-level detail that those files are
backed by swap.

> Rss value decreases also when swap is disabled, so this is where I am
> lost as how shared memory is supposed to behave.

Did you check that detail on both 2.6.28 and 2.6.31-rc4?  I think
2.6.28 was unmapping the ptes from the pagetables, before the lower
level found that it had no swap to write them to; whereas a current
kernel didn't unmap them at all in my case.

> 
> I have test program which makes 32MB shared memory segment and then I
> use 'stress -m 1 --vm-bytes 120M', --vm-bytes is increased until rss
> size decreases in smaps. Swap value never increases in smaps.
> 
> On the other hand shmctl(0, SHM_INFO, ...) does show shared memory in
> swap because shm.c shm_get_stat() uses inodes to get values.

SHM-specific tools know they're dealing with tmpfs and perhaps swap,
and so can present a more tailored version of the info.

Hugh

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