Re: Kernel Benchmarking

On Sun, Sep 13, 2020 at 10:40:57AM +1000, Dave Chinner wrote:

quoted

The reason the apache benchmark regresses is that it basically does a
web server test with a single file ("test.html") that gets served by
just mmap'ing it, and sending it out that way. Using lots of threads,
and using lots of different mappings. So they *all* fault on the read
of that page, and they *all* do that "lock page, check that the
mapping is valid, insert page" dance.

Hmmmm. So this is a typically a truncate race check, but this isn't
sufficient to protect the fault against all page invalidation races
as the page can be re-inserted into the same mapping at a different
page->index now within EOF.

No it can't.  find_get_page() returns the page with an elevated refcount.
The page can't be reused until we call put_page().  It can be removed
from the page cache, but can't go back to the page allocator until the
refcount hits zero.

5) filesystems will still need to be able to exclude page faults
over a file range while they directly manipulate file metadata to
change the user data in the file

Yes, but they can do that with a lock inside ->readpage (and, for that
matter in ->readahead()), so there's no need to take a lock for pages
which are stable in cache.