On 23.01.2018 22:15, David Hildenbrand wrote: > On 13.12.2017 13:53, Janosch Frank wrote: > Please correct me if I'm wrong (this stuff is complicated): > > > Right now we have to split huge pages under the following condition: > > a) We are write protecting (prot != PROT_WRITE) ... > b) ... and we are doing it during shadow page table creation > (GMAP_NOTIFY_SHADOW) > > -> gmap_protect_pmd() Yes > > > This is to work around issues (RW vs. RO) when > a) G2 puts G2->G3 DAT tables on same huge page as a G2 prefix > b) Guest G2->G3 DAT tables on same huge page as G2->G3 pages referenced > in such a table > > "we cannot have RO and RW at the same time if things depend on each other". Yes > > > Now, the interesting thing is, for shadow page tables > (GMAP_NOTIFY_SHADOW), we only protect RO: via gmap_protect_rmap() and > gmap_protect_range(). > > So basically for all shadow page table housekeeping, we never protect on > pmds but only on ptes. -> We always split huge pages > > This implies and important insight: _SEGMENT_ENTRY_GMAP_VSIE is never > used. (and I will prepare a cleanup patch to make PROT_READ implicit on > e.g. gmap_protect_rmap(), because this clarifies this a lot) Yes, I guess _SEGMENT_ENTRY_GMAP_VSIE is a leftover from before the splitting. > > > We only ever protect right now on huge pages without splitting it up for > the prefix, as I already mentioned. And as discussed, I doubt this is > really worth it. And we can get rid of a lot of code this way. See next answer > > > Long story short: > > If we simply split up huge pages when protecting the prefix, we don't > need gmap_protect_pmd() anymore, and therefore also (at least) not We need it for the dirty tracking, no? > > - s390/mm: Abstract gmap notify bit setting Yes, that's not needed then. > - s390/mm: add gmap PMD invalidation notification We need that one (in parts) because of the protection transfer to user space. We will be notified on mm pmds. Even if we split a pmd, we will be notified on a pmd, not on a pte. So we need at least a skeleton that calls pmdp_notify_split. I'm currently preparing a patch that rips out pmd protection with software bits. I'll attach it when finished, so we can have a look what can go. > > > So I think doing proper sub-hugepage protection right from the beginning > makes perfect sense. > > @Martin, Christian, am I missing something? What's your take on this? >
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