Re: [PATCH] add extra free kbytes tunable

[Hugh Dickins <hughd@google.com>]

On Sat, 2 Mar 2013, Simon Jeons wrote:
> 
> In function __add_to_swap_cache if add to radix tree successfully will result
> in increase NR_FILE_PAGES, why? This is anonymous page instead of file backed
> page.

Right, that's hard to understand without historical background.

I think the quick answer would be that we used to (and still do) think
of file-cache and swap-cache as two halves of page-cache.  And then when
someone changed the way stats were gathered, they couldn't very well
name the stat for page-cache pages NR_PAGE_PAGES, so they called it
NR_FILE_PAGES - but it still included swap.

We have tried down the years to keep the info shown in /proc/meminfo
(for example, but it is the prime example) consistent across releases,
while adding new lines and new distinctions.

But it has often been hard to find good enough short enough names for
those new distinctions: when 2.6.28 split the LRUs between file-backed
and swap-backed, it used "anon" for swap-backed in /proc/meminfo.

So you'll find that shmem and swap are counted as file in some places
and anon in others, and it's hard to grasp which is where and why,
without remembering the history.

I notice that fs/proc/meminfo.c:meminfo_proc_show() subtracts
total_swapcache_pages from the NR_FILE_PAGES count for /proc/meminfo:
so it's undoing what you observe __add_to_swap_cache() to be doing.

It's quite possible that if you went through all the users of
NR_FILE_PAGES, you'd find it makes much more sense to leave out
the swap-cache pages, and just add those on where needed.

But you might find a few places where it's hard to decide whether
the swap-cache pages were ever intended to be included or not, and
hard to decide if it's safe to change those numbers now or not.

Hugh