On Fri, Jan 15, 2021 at 12:08:37PM -0500, Peter Xu wrote: > This is a RFC series to support userfaultfd upon shmem and hugetlbfs. > > PS. Note that there's a known issue [0] with tlb against uffd-wp/soft-dirty in > general and Nadav is working on it. It may or may not directly affect > shmem/hugetlbfs since there're no COW on shared mappings normally. Private > shmem could hit, but still that's another problem to solve in general, and this > RFC is majorly to see whether there's any objection on the concept of the idea > specific to uffd-wp on shmem/hugetlbfs. > > The whole series can also be found online [1]. > > The major comment I'd like to get is on the new idea of swap special pte. That > comes from suggestions from both Hugh and Andrea and I appreciated a lot for > those discussions. > > In short, it's a new type of pte that doesn't exist in the past, while used in > file-backed memories to persist information across ptes being erased (but the > page cache could still exist, for example, so in the next page fault we can > reload the page cache with that specific information when necessary). > > I'm copy-pasting some commit message from the patch "mm/swap: Introduce the > idea of special swap ptes", where uffd-wp becomes the first user of it: > > We used to have special swap entries, like migration entries, hw-poison > entries, device private entries, etc. > > Those "special swap entries" reside in the range that they need to be at least > swap entries first, and their types are decided by swp_type(entry). > > This patch introduces another idea called "special swap ptes". > > It's very easy to get confused against "special swap entries", but a speical > swap pte should never contain a swap entry at all. It means, it's illegal to > call pte_to_swp_entry() upon a special swap pte. > > Make the uffd-wp special pte to be the first special swap pte. > > Before this patch, is_swap_pte()==true means one of the below: > > (a.1) The pte has a normal swap entry (non_swap_entry()==false). For > example, when an anonymous page got swapped out. > > (a.2) The pte has a special swap entry (non_swap_entry()==true). For > example, a migration entry, a hw-poison entry, etc. > > After this patch, is_swap_pte()==true means one of the below, where case (b) is > added: > > (a) The pte contains a swap entry. > > (a.1) The pte has a normal swap entry (non_swap_entry()==false). For > example, when an anonymous page got swapped out. > > (a.2) The pte has a special swap entry (non_swap_entry()==true). For > example, a migration entry, a hw-poison entry, etc. > > (b) The pte does not contain a swap entry at all (so it cannot be passed > into pte_to_swp_entry()). For example, uffd-wp special swap pte. > > Hugetlbfs needs similar thing because it's also file-backed. I directly reused > the same special pte there, though the shmem/hugetlb change on supporting this > new pte is different since they don't share code path a lot. Huge & Mike, Would any of you have comment/concerns on the high-level design of this series? It would be great to know it, especially major objection, before move on to an non-rfc version. Thanks, -- Peter Xu
