> Am 17.12.2019 um 19:25 schrieb Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>: > > On Tue, 2019-12-17 at 18:24 +0100, David Hildenbrand wrote: >>>>> Also there are some scenarios where __page_to_pfn is not that simple a >>>>> call with us having to get the node ID so we can find the pgdat structure >>>>> to perform the calculation. I'm not sure the compiler would be ble to >>>>> figure out that the result is the same for both calls, so it is better to >>>>> make it explicit. >>>> >>>> Only in case of CONFIG_SPARSEMEM we have to go via the section - but I >>>> doubt this is really worth optimizing here. >>>> >>>> But yeah, I'm fine with this change, only "IMHO >>>> get_pageblock_migratetype() would be nicer" :) >>> >>> Aren't most distros running with CONFIG_SPARSEMEM enabled? If that is the >>> case why not optimize for it? >> >> Because I tend to dislike micro-optimizations without performance >> numbers for code that is not on a hot path. But I mean in this case, as >> you said, you need the pfn either way, so it's completely fine with. >> >> I do wonder, however, if you should just pass in the migratetype from >> the caller. That would be even faster ;) > > The problem is page isolation. We can end up with a page being moved to an > isolate pageblock while we aren't holding the zone lock, and as such we > likely need to test it again anyway. So there isn't value in storing and > reusing the value for cases like page reporting. > > In addition, the act of isolating the page can cause the migratetype to > change as __isolate_free_page will attempt to change the migratetype to > movable if it is one of the standard percpu types and we are pulling at > least half a pageblock out. So storing the value before we isolate it > would be problematic as well. > > Undoing page isolation is the exception to the issues pointed out above, > but in that case we are overwriting the pageblock migratetype anyway so > the cache lines involved should all be warm from having just set the > value. Nothing would speak against querying the migratetype in the caller and passing it on. After all you‘re holding the zone lock, so it can‘t change. >
