On Tue, 2019-12-17 at 19:46 +0100, David Hildenbrand wrote: > > 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. That's a fair argument. I will go ahead and make that change since it only really adds one line to patch 4 and allows us to drop several lines from patch 3.
