On 30.06.20 00:58, Wei Yang wrote: > On Mon, Jun 29, 2020 at 03:13:25PM -0700, Dan Williams wrote: >> On Mon, Jun 29, 2020 at 1:34 AM Wei Yang >> <richard.weiyang@xxxxxxxxxxxxxxxxx> wrote: >>> >>> On Thu, Jun 25, 2020 at 12:46:43PM -0700, Dan Williams wrote: >>>> On Wed, Jun 24, 2020 at 10:53 PM David Hildenbrand <david@xxxxxxxxxx> wrote: >>>>> >>>>> >>>>> >>>>>> Am 25.06.2020 um 01:47 schrieb Dan Williams <dan.j.williams@xxxxxxxxx>: >>>>>> >>>>>> On Wed, Jun 24, 2020 at 3:44 PM Wei Yang >>>>>> <richard.weiyang@xxxxxxxxxxxxxxxxx> wrote: >>>>>> [..] >>>>>>>> So, you are right that there is a mismatch here, but I think the >>>>>>>> comprehensive fix is to allow early sections to be partially >>>>>>>> depopulated/repopulated rather than have section_activate() and >>>>>>>> section_deacticate() special case early sections. The special casing >>>>>>>> is problematic in retrospect as section_deactivate() can't be >>>>>>>> maintained without understand special rules in section_activate(). >>>>>>> >>>>>>> Hmm... This means we need to adjust pfn_valid() too, which always return true >>>>>>> for early sections. >>>>>> >>>>>> Right, rather than carry workarounds in 3 locations, and the bug that >>>>>> has resulted from then getting out of sync, just teach early section >>>>>> mapping to allow for the subsection populate/depopulate. >>>>>> >>>>> >>>>> I prefer the easy fix first - IOW what we Here here. Especially, pfn_to_online_page() will need changes as well. >>>> >>>> Agree, yes, let's do the simple fix first for 5.8 and the special-case >>>> elimination work later. >>> >>> Dan, >>> >>> A quick test shows this is not a simple task. >> >> Thanks for taking a look... >> >>> First, early sections don't set subsection bitmap, which is necessary for the >>> hot-add/remove. >>> >>> To properly set subsection bitmap, we need to know how many subsections in >>> early section. While current code doesn't has a alignment requirement for >>> last early section. We mark the whole last early section as present. >> >> I was thinking that the subsection map does not need to be accurate on >> initial setup, it only needs to be accurate after the first removal. >> However, that would result in new special casing that somewhat defeats >> the purpose. The hardest part is potentially breaking up a PMD mapping >> of the page array into a series of PTE mappings without disturbing >> in-flight pfn_to_page() users. >> >>> I don't find a way to enable this. >> >> While I don't like that this bug crept into the mismatched special >> casing of early sections, I'm now coming around to the same opinion. >> I.e. that making the memmap for early sections permanent is a simpler >> mechanism to maintain. > > I think so ... > Yes, and I think having to replace quite some pfn_valid_within() - nops - by pfn_valid() just to handle one corner case might not be worth it. At least for now. -- Thanks, David / dhildenb
