On Mon, 9 Jun 2014, Vlastimil Babka wrote: > > > > Sorry, I meant ACCESS_ONCE(page_private(page)) in the migration scanner > > > > > > Hm but that's breaking the abstraction of page_order(). I don't know if > > > it's > > > worse to create a new variant of page_order() or to do this. BTW, seems > > > like > > > next_active_pageblock() in memory-hotplug.c should use this variant too. > > > > > > > The compiler seems free to disregard the access of a volatile object above > > because the return value of the inline function is unsigned long. What's > > the difference between unsigned long order = page_order_unsafe(page) and > > unsigned long order = (unsigned long)ACCESS_ONCE(page_private(page)) and > > I think there's none functionally, but one is abstraction layer violation and > the other imply the context of usage as you say (but is that so uncommon?). > > > the compiler being able to reaccess page_private() because the result is > > no longer volatile qualified? > > You think it will reaccess? That would defeat all current ACCESS_ONCE usages, > no? > I think the compiler is allowed to turn this into if (ACCESS_ONCE(page_private(page)) > 0 && ACCESS_ONCE(page_private(page)) < MAX_ORDER) low_pfn += (1UL << ACCESS_ONCE(page_private(page))) - 1; since the inline function has a return value of unsigned long but gcc may not do this. I think /* * Big fat comment describing why we're using ACCESS_ONCE(), that * we're ok to race, and that this is meaningful only because of * the previous PageBuddy() check. */ unsigned long pageblock_order = ACCESS_ONCE(page_private(page)); is better. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>