Hi Hugh,
On 03/02/2013 11:08 AM, Hugh Dickins wrote:
On Sat, 2 Mar 2013, Simon Jeons wrote:
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.
A shmem page is swap-backed file-cache, and it may get transferred to or
from swap-cache: yes, it's a difficult and confusing case, as I said below.
I would never call it "anonymous", but it is counted in /proc/meminfo's
Active(anon) or Inactive(anon) rather than in (file), because "anon"
there is shorthand for "swap-backed".
In read_swap_cache_async:
SetPageSwapBacked(new_page);
__add_to_swap_cache();
swap_readpage();
ClearPageSwapBacked(new_page);
Why clear PG_swapbacked flag?
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.
Hugh
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