Re: [PATCH] add extra free kbytes tunable

[Simon Jeons <simon.jeons@gmail.com>]

On 03/02/2013 09:42 AM, Hugh Dickins wrote:
> 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

shmem page should be treated as file-cache or swap-cache? It is strange 
since it is consist of anonymous pages and these pages establish files.

> 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

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