On 24.04.20 09:55, Vlastimil Babka wrote: > On 4/24/20 9:28 AM, David Hildenbrand wrote: >> On 24.04.20 02:41, Matthew Wilcox wrote: >>> On Thu, Apr 23, 2020 at 05:37:00PM -0700, Andrew Morton wrote: >>>> On Wed, 22 Apr 2020 16:09:00 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote: >>>>> Heh, I was quite sure that this is not the first time background zeroing is >>>>> proposed, so I went to google for it... and found that one BSD kernel actually >>>>> removed this functionality in 2016 [1] and this was one of the reasons. >>>>> >>>>> [1] >>>>> https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/afd2da4dc9056ea79cdf15e8a9386a3d3998f33e >>>> >>>> Interesting. >>>> >>>> However this: >>>> >>>> - Pre-zeroing a page leads to a cold-cache case on-use, forcing the fault >>>> source (e.g. a userland program) to actually get the data from main >>>> memory in its likely immediate use of the faulted page, reducing >>>> performance. >>>> >>>> implies that BSD was zeroing with non-temporal stores which bypass the >>>> CPU cache. And which presumably invalidate any part of the target >>>> memory which was already in cache. We wouldn't do it that way so >>>> perhaps the results would differ. >>> >>> Or just that the page was zeroed far enough in advance that it fell out >>> of cache naturally. > > Agreed. > >>> I know Arjan looked at zeroing on free instead of zeroing on alloc, >>> and that didn't get merged (or even submitted afaik), so presumably the >>> results weren't good. >> >> We do have INIT_ON_FREE_DEFAULT_ON >> >> via >> >> commit 6471384af2a6530696fc0203bafe4de41a23c9ef >> Author: Alexander Potapenko <glider@xxxxxxxxxx> >> Date: Thu Jul 11 20:59:19 2019 -0700 >> >> mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options >> >> which seems to do exactly that (although for a different use case) > > Yeah, except the security use case wants to do that immediately, while the > proposal here is a background thread. > Yes I know, this was just a comment regarding "Arjan looked at zeroing on free instead of zeroing on alloc". -- Thanks, David / dhildenb
