On Thu, Aug 04, 2011 at 02:07:21PM -0700, Michel Lespinasse wrote: > This change introduces a new lock in order to simplify the way > __split_huge_page_refcount and put_compound_page interact. > > The synchronization problem in this code is that when operating on > tail pages, put_page() needs to adjust page counts for both the tail > and head pages. On the other hand, when splitting compound pages > __split_huge_page_refcount() needs to adjust the head page count so that > it does not reflect tail page references anymore. When the two race > together, they must agree as to the order things happen so that the head > page reference count does not end up with an improper value. > > I propose doing this using a new lock on the tail page. Compared to > the previous version using the compound lock on the head page, > the compound page case of put_page() ends up being much simpler. > > The new lock is implemented using the lowest bit of page->_count. > Page count accessor functions are modified to handle this transparently. > New accessors are added in mm/internal.h to lock/unlock the > page count lock while simultaneously accessing the page count value. > The number of atomic operations required is thus minimized. > > Note that the current implementation takes advantage of the implicit > memory barrier provided by x86 on atomic RMW instructions to provide > the expected lock/unlock semantics. Clearly this is not portable > accross architectures, and will have to be accomodated for using > an explicit memory barrier on architectures that require it. > > Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx> I didn't take a long time to find out any faults but I see the approach and it seems no problem except barrier stuff. I agree this patch makes simple thing complicated by THP in put_page. It would be very good about readability. :) But the concern is that put_page on tail page is rare operation but get_page is very often one. And you are going to enhance readability as scarificing the performance. A shift operation cost would be negligible but at least we need the number. If it doesn't hurt performance, I absolutely support your patch!. Because your patch would reduce many atomic opeartion on head page of put_page as well as readbility. -- Kind regards, Minchan Kim -- 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>