Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings

[David Hildenbrand]

On 11.07.24 19:57, Linus Torvalds wrote:
> On Thu, 11 Jul 2024 at 10:09, Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> > When I was working on this patchset this year with the syscall, this is
> > similar somewhat to the initial approach I was taking with setting up a
> > special mapping. It turned into kind of a mess and I couldn't get it
> > working. There's a lot of functionality built around anonymous pages
> > that would need to be duplicated (I think?).
>
> Yeah, I was kind of assuming that. You'd need to handle VM_DROPPABLE
> in the fault path specially, the way we currently split up based on
> vma_is_anonymous(), eg
>
>          if (vma_is_anonymous(vmf->vma))
>                  return do_anonymous_page(vmf);
>          else
>                  return do_fault(vmf);
>
> in do_pte_missing() etc.
>
> I don't actually think it would be too hard, but it's a more
> "conceptual" change, and it's probably not worth it.
>
> > Alright, an hour later of fiddling, and it doesn't actually work (yet?)
> > -- the selftest fails. A diff follows below.
>
> May I suggest a slightly different approach: do what we did for "pte_mkwrite()".
>
> It needed the vma too, for not too dissimilar reasons: special dirty
> bit handling for the shadow stack. See
>
>    bb3aadf7d446 ("x86/mm: Start actually marking _PAGE_SAVED_DIRTY")
>    b497e52ddb2a ("x86/mm: Teach pte_mkwrite() about stack memory")
>
> and now we have "pte_mkwrite_novma()" with the old semantics for the
> legacy cases that didn't get converted - whether it's because the
> architecture doesn't have the issue, or because it's a kernel pte.
>
> And the conversion was actually quite pain-free, because we have
>
>    #ifndef pte_mkwrite
>    static inline pte_t pte_mkwrite(pte_t pte, struct vm_area_struct *vma)
>    {
>          return pte_mkwrite_novma(pte);
>    }
>    #endif
>
> so all any architecture that didn't want this needed to do was to
> rename their pte_mkwrite() to pte_mkwrite_novma() and they were done.
> In fact, that was done first as basically semantically no-op patches:
>
>     2f0584f3f4bd ("mm: Rename arch pte_mkwrite()'s to pte_mkwrite_novma()")
>     6ecc21bb432d ("mm: Move pte/pmd_mkwrite() callers with no VMA to _novma()")
>     161e393c0f63 ("mm: Make pte_mkwrite() take a VMA")
>
> which made this all very pain-free (and was largely a sed script, I think).
>
> > -                   !pte_dirty(pte) && !PageDirty(page))
> > +                   !pte_dirty(pte) && !PageDirty(page) &&
> > +                   !(vma->vm_flags & VM_DROPPABLE))
>
> So instead of this kind of thing, we'd have
>
> > -                   !pte_dirty(pte) && !PageDirty(page))
> > +                   !pte_dirty(pte, vma) && !PageDirty(page) &&
>
> and the advantage here is that you can't miss anybody by mistake. The
> compiler will be very unhappy if you don't pass in the vma, and then
> any places that would be converted to "pte_dirty_novma()"
>
> We don't actually have all that many users of pte_dirty(), so it
> doesn't look too nasty. And if we make the pte_dirty() semantics
> depend on the vma, I really think we should do it the same way we did
> pte_mkwrite().

We also have these folio_mark_dirty() calls, for example in 
unpin_user_pages_dirty_lock(). Hm ... so preventing the folio from 
getting dirtied is likely shaky.

I guess we need a way to just reliably identify these folios :/.

--
Cheers,

David / dhildenb