On 07/13/2016 08:21 AM, Michal Hocko wrote: >> > This adds a tiny amount of overhead to all pte_none() checks. >> > I doubt we'll be able to measure it anywhere. > It would be better to introduce the overhead only for the affected > cpu models but I guess this is also acceptable. Would it be too > complicated to use alternatives for that? The patch as it stands ends up doing a one-instruction change in pte_none(). It goes from 64c8: 48 85 ff test %rdi,%rdi to 64a8: 48 f7 c7 9f ff ff ff test $0xffffffffffffff9f,%rdi So it essentially eats 4 bytes of icache more than it did before. But, it's the same number of instructions, and I can't imagine that the CPU will have any more trouble with a test against an immediate than a test against 0. We could theoretically do alternatives for this, but we would at *best* end up with 4 bytes of noops. So, unless the processor likes decoding 4 noops better than 4 bytes of immediate as part of an instruction, we'll not win anything. *Plus* the ugliness of the assembly that we'll need to have the compiler guarantee that the PTE ends up in %rdi. -- 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>