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 ... -- Wei Yang Help you, Help me
