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In that case >>> the direct map will probably be carved up pretty quick. Did you >>> consider just breaking the full direct map to 4k at the start when >>> it's in use? >> >> That's an interesting point, I haven't thought about it from this >> perspective. We should run some tests internally to see if it'd >> help. This will likely change with support for huge pages coming in >> though. > > The thing is, those no_flush() helpers actually still flush if they > need to split a page. Plus if they need to clear out lazy vmalloc > aliases it could be another flush. There are probably a lot of > opportunities to reduce flushing even beyond pre-split. > > Just curious... as far as performance, have you tested this on a big > multi-socket system, where that flushing will hurt more? It's something > that has always been a fear for these directmap unmapping solutions Yes, this is a problem that we'd like to address. We have been discussing it in [1]. The effect of flushing on memory population that we see on x86 is 5-7x elongation. We are thinking of making use of the no-direct-map memory allocator that Brendan is working on [2]. [1] https://lore.kernel.org/lkml/d1b58114-9b88-4535-b28c-09d9cc1ff3be@amazon.com [2] https://lore.kernel.org/kvm/DDVS9ITBCE2Z.RSTLCU79EX8G@google.com >