Re: meminfo returns inaccurate NR_FILE_PAGES

[Ethan Solomita <solo@google.com>]

Christoph Lameter wrote:
>> It's just the NR_FILE_PAGES which are tied to the mapping's page tree, where
>> the problem lies.
>>     
>
> Ah. I see.
>
> However, anonymous pages may also have a mapping (swap). So we need to 
> check first that it is not an anonymous page and then eventually shift 
> the count between zones.
>   

    Anonymous pages have a value in mapping, but it's not a struct 
address_space, it's a struct vm_area_struct (+1). The NR_FILE_PAGES 
count is incremented and decremented only when something is added to or 
removed from an address_space's page_table as pointed to by a mapping. 
This is only done in filemap.c, except for this one example in migrate.c 
that changes the radix table's page pointer in place. I think that all 
that is needed is an extra set of lines in migrate_page_move_mapping() 
after modifying *radix_pointer to call __dec on the old page and __inc 
on the new. You can check the zones first if you'd like to save effort, 
although I'm not sure it's a big deal since the __dec and __inc 
functions are only modifying per-cpu accumulation variables.
    -- Ethan

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