Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI

[Peter Xu]

On Thu, Sep 28, 2023 at 07:05:40PM +0200, David Hildenbrand wrote:
> As described as reply to v1, without fork() and KSM, the PAE bit should
> stick around. If that's not the case, we should investigate why.
> 
> If we ever support the post-fork case (which the comment above remap_pages()
> excludes) we'll need good motivation why we'd want to make this
> overly-complicated feature even more complicated.

The problem is DONTFORK is only a suggestion, but not yet restricted.  If
someone reaches on top of some !PAE page on src it'll never gonna proceed
and keep failing, iiuc.

do_wp_page() doesn't have that issue of accuracy only because one round of
CoW will just allocate a new page with PAE set guaranteed, which is pretty
much self-heal and unnoticed.

So it'll be great if we can have similar self-heal way for PAE.  If not, I
think it's still fine we just always fail on !PAE src pages, but then maybe
we should let the user know what's wrong, e.g., the user can just forgot to
apply DONTFORK then forked.  And then the user hits error and don't know
what happened.  Probably at least we should document it well in man pages.

Another option can be we keep using folio_mapcount() for pte, and another
helper (perhaps: _nr_pages_mapped==COMPOUND_MAPPED && _entire_mapcount==1)
for thp.  But I know that's not ideal either.

-- 
Peter Xu