Re: [PATCH 0/1] mm: Optimizing hugepage zeroing in arm64

From: Robin Murphy
Date: Thu Jan 21 2021 - 14:10:02 EST

On 2021-01-21 17:46, Will Deacon wrote:
On Thu, Jan 21, 2021 at 10:21:50PM +0530, Prathu Baronia wrote:
This patch removes the unnecessary kmap calls in the hugepage zeroing path and
improves the timing by 62%.

I had proposed a similar change in Apr-May'20 timeframe in memory.c where I
proposed to clear out a hugepage by directly calling a memset over the whole
hugepage but got the opposition that the change was not architecturally neutral.

Upon revisiting this now I see significant improvement by removing around 2k
barrier calls from the zeroing path. So hereby I propose an arm64 specific
definition of clear_user_highpage().

Given that barrier() is purely a thing for the compiler, wouldn't the same
change yield a benefit on any other architecture without HIGHMEM? In which
case, I think this sort of change belongs in the core code if it's actually

I would have thought it's more the constant manipulation of the preempt and pagefault counts, rather than the compiler barriers between them, that has the impact. Either way, if arm64 doesn't need to be atomic WRT preemption when clearing parts of hugepages then I also can't imagine that anyone else (at least for !HIGHMEM) would either.