Re: [PATCH] filemap: add trace events for get_pages, map_pages, and fault

[Takaya Saeki <takayas@chromium.org>]

Thanks Steven for trying it out.

> I can see it bringing down the number of pages needed to be saved dramatically.

Yes, I agree.
However, note that wc does not count the size of the page caches
correctly since 'get_map_pages' gives you a range. In your example of
the less command, actually the total sizes of page_cache_size and
get_map_pages look the same.

Instead of less, running a large binary such as Chrome (232MB) gives
us a better example. '# trace-cmd record -e filemap chrome --version'
showed 58% reduction from 42MB to 17MB in my environment.
- Total size of mm_filemap_add_to_page_cache: 42,958,848 bytes
- With mm_filemap_map_pages and mm_filemap_get_pages: 17,993,728 bytes

By the way,  'mm_filemap_map_pages' traces a range requested to
populate, which includes pages that are not in caches yet and thus are
skipped. So, you need to calculate the intersection with
mm_filemap_add_to_page_cache to see page caches that are actually
mapped.
I'm wondering if we should put a trace event per each successful page
mapping event as mm_filemap_add_to_page_cache does, by putting an
event inside the page map loop.