Re: [PATCH v2] mm: Don't fault around userfaultfd-registered regions on reads

[Peter Xu]

On Wed, Dec 02, 2020 at 02:37:33PM -0800, Hugh Dickins wrote:
> On Tue, 1 Dec 2020, Andrea Arcangeli wrote:
> > 
> > Any suggestions on how to have the per-vaddr per-mm _PAGE_UFFD_WP bit
> > survive the pte invalidates in a way that remains associated to a
> > certain vaddr in a single mm (so it can shoot itself in the foot if it
> > wants, but it can't interfere with all other mm sharing the shmem
> > file) would be welcome...
> 
> I think it has to be a new variety of swap-like non_swap_entry() pte,
> see include/linux/swapops.h.  Anything else would be more troublesome.
> 
> Search for non_swap_entry and for migration_entry, to find places that 
> might need to learn about this new variety.
> 
> IIUC you only need a single value, no need to carve out another whole
> swp_type: could probably be swp_offset 0 of any swp_type other than 0.
> 
> Note that fork's copy_page_range() does not "copy ptes where a page
> fault will fill them correctly", so would in effect put a pte_none
> into the child where the parent has this uffd_wp entry.  I don't know
> anything about uffd versus fork, whether that would pose a problem.

Thanks for the idea, Hugh!

I thought about something similar today, but instead of swap entries, I was
thinking about constantly filling in a pte with a value of "_PAGE_PROTNONE |
_PAGE_UFFD_WP" when e.g. we'd like to zap a page with shmem+uffd-wp. I feel
like the fundamental idea is similar - we can somehow keep the pte with uffd-wp
information even if zapped/swapped-out, so as long as the shmem access will
fruther trap into the fault handler, then we can operate on that pte and read
that information out, like recover that pte into a normal pte (with swap/page
cache, and vma/addr information, we'll be able to) and then we can retry the
fault.

Thanks,

-- 
Peter Xu