Re: [PATCH] kernel: fs: drop_caches: add dds drop_caches_count

[Dave Chinner <david@fromorbit.com>]

On Mon, Feb 15, 2016 at 09:57:42PM -0800, Nag Avadhanam wrote:
> On Mon, 15 Feb 2016, Dave Chinner wrote:
> >So, to pick a random active server here:
> >
> >		before		after
> >Active(file):   12103200 kB	24060 kB
> >Inactive(file):  5976676 kB	 1380 kB
> >Mapped:            31308 kB	31308 kB
> >
> >How much was not reclaimed? Roughly the same number of pages as the
> >Mapped count, and that's exactly what we'd expect to see from the
> >above page walk counting code. Hence a slightly better approximation
> >of the pages that dropping caches will reclaim is:
> >
> >reclaimable pages = active + inactive - dirty - writeback - mapped
> 
> Thanks Dave. I considered that, but see this.
> 
> Mapped page count below is much higher than the (active(file) +
> inactive (file)).

Yes. it's all unreclaimable from drop caches, though.

> Mapped seems to include all page cache pages mapped into the process
> memory, including the shared memory pages, file pages and few other
> type
> mappings.
> 
> I suppose the above can be rewritten as (mapped is still high):
> 
> reclaimable pages = active + inactive + shmem - dirty - writeback - mapped
> 
> What about kernel pages mapped into user address space? Does "Mapped"
> include those pages as well? How do we exclude them? What about
> device mappings? Are these excluded in the "Mapped" pages
> calculation?

/me shrugs.

I have no idea - I really don't care about what pages are accounted
as mapped. I assumed that the patch proposed addressed your
requirements and so I suggested an alternative that provided almost
exactly the same information but erred on the side of
underestimation and hence solves your problem of drop_caches not
freeing as much memory as you expected....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com