Re: [rfc] no ZERO_PAGE?

[Andrea Arcangeli <andrea@suse.de>]

On Wed, Apr 04, 2007 at 04:55:32PM +0300, Dan Aloni wrote:
> How about applications that perform mmap() and R/W random-access on 
> large *sparse* files? (e.g. a scientific app that uses a large sparse 
> file as a big database look-up table). As I see it, these apps would
> need to keep track of what's sparse and what's not...

That's not anonymous memory if those are read page faults on
_files_. I'm only talking about anonymous memory and
do_anonymous_page, i.e. no file data at all. In more clear words, the
only thing we're discussing here is char = malloc(1); *char.

Your example _already_ allocates zeroed pagecache instead of the zero
page, so your example (random access over sparse files with mmap, be
it MAP_PRIVATE or MAP_SHARED no difference for reads) has never had
anything to do with the zero page. If something we could optimize your
example to _start_ using for the first time ever the ZERO_PAGE, it
would make more sense to use it to be mapped where the lowlevel fs
finds holes. ZERO_PAGE in do_anonymous_page instead doesn't make much
sense to me, but it has always been there as far as I can
remember. The thing is that it never hurted until the huge systems
with nightmare cacheline bouncing reported heavy stalls on some
testcase, which make it look like a DoS because of the ZERO_PAGE,
hence now that it hurts I guess it can go.