On Fri, Sep 02, 2011 at 01:45:27AM +0200, Andi Kleen wrote:

On Thu, Sep 01, 2011 at 04:28:08PM -0700, Andrew Morton wrote:

quoted

On Thu, 1 Sep 2011 17:24:17 +0200
Andrea Arcangeli [off-list ref] wrote:

quoted

Ideally direct-io should stop calling get_page() on pages
returned by get_user_pages().

Yeah.  get_user_pages() is sufficient.  Ideally we should be able to
undo the get_user_pages() get_page() from within the IO completion
interrupt and we're done.

Cc Andi, who is our resident dio tweaker ;)

Noted, I'll put it on my list.

Thanks Andi!

Should not be too difficult from a quick look, just the convoluted
nature of direct-io.c requires a lot of double checking.

I also had a look but it wasn't trivial, I'm not even sure why
direct-io.c has to be convoluted.

If we could optimize that, we would stay within get_page_foll() which
won't need to take the compound_lock even for tail
pages. (compound_lock can't be avoided for put_page on tail pages
because it runs long after we release any VM lock)

Calling get_page/put_pages more times than necessary is never ideal, I
imagine the biggest cost is the atomic_inc on the head page that
brings in the cacheline of the head page exclusive, the compound_lock
in the second get_page shouldn't have a measurable effect, so I think
from a practical prospective it's not more worthwhile to optimize
that now, than it already was before.

Cc Andi, who is our resident dio tweaker ;)

Thanks :)

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>