Re: [PATCH RFC 00/30] userfaultfd-wp: Support shmem and hugetlbfs

[Peter Xu <peterx@redhat.com>]

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