Hi Andrea, On Mon, Aug 22, 2011 at 2:33 PM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote: > So this solution: > > 1) should allow the working set estimation code to keep doing its > get_page_unless_zero() without any change (you'll still have to modify > it to check if you got a THP page etc... but you won't risk to get any > tail page anymore). Maybe it still needs some non trivial thought > about the changes but not anymore about tail pages refcounting screwups. > > 2) no change to all existing get_page_unless_zero() is required, so > this should fix the radix tree speculative page lookup too. > > 3) no RCU new feature is needed Adding Paul McKenney so he won't spend too much time on RCU cookie feature until there is a firmer user... > 4) get_page was actually called by direct-io as my debug > instrumentation I wrote to test these changes noticed it so I fixed > that too Looks like this scheme will work. I'm off in Yosemite for a few days with my family, but I should be able to review this more thoroughly on Thursday. >From a few-minutes look, I have a few minor concerns: - When splitting THP pages, the old tail refcount will be visible as the _mapcount for a short while after PageTail is cleared; not clear yet to me if there are unintended side effects to that; - (not a concern, but an opportunity) when splitting pages, there are two atomic adds to the tail _count field, while we know the initial value is 0. Why not just one straight assignment ? Similarly, the adjustments to page head count could be added into a local variable and the page head count could be updated once after all tail pages have been split off. - Not sure if we could/should add assertions to make sure people call the right get_page variant. The other question I have is about the use of the pagemap.h RCU protocol for eventual page count stability. With your proposal, this would now affect only head pages, so THP splitting is fine :) . I'm not sure who else might use that protocol, but it looks like we should either make all get_pages_unless_zero call sites follow it (if the protocol matters to someone) or none (if the protocol turns out to be obsolete). Sorry for the incomplete reply, I'll have a better one by Thursday :) Thanks, -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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@xxxxxxxxx";> email@xxxxxxxxx </a>