Re: [PATCH v3 11/27] shmem/userfaultfd: Persist uffd-wp bit across zapping for file-backed

Peter Xu
2021-07-06T15:35:28Z

On Tue, Jul 06, 2021 at 03:40:42PM +1000, Alistair Popple wrote:
> > > > > > > > struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> > > > > > > > pte_t pte);
> > > > > > > > struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
> > > > > > > > diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> > > > > > > > index 355ea1ee32bd..c29a6ef3a642 100644
> > > > > > > > --- a/include/linux/mm_inline.h
> > > > > > > > +++ b/include/linux/mm_inline.h
> > > > > > > > @@ -4,6 +4,8 @@
> > > > > > > >
> > > > > > > > #include <linux/huge_mm.h>
> > > > > > > > #include <linux/swap.h>
> > > > > > > > +#include <linux/userfaultfd_k.h>
> > > > > > > > +#include <linux/swapops.h>
> > > > > > > >
> > > > > > > > /**
> > > > > > > > * page_is_file_lru - should the page be on a file LRU or anon LRU?
> > > > > > > > @@ -104,4 +106,45 @@ static __always_inline void del_page_from_lru_list(struct page *page,
> > > > > > > > update_lru_size(lruvec, page_lru(page), page_zonenum(page),
> > > > > > > > -thp_nr_pages(page));
> > > > > > > > }
> > > > > > > > +
> > > > > > > > +/*
> > > > > > > > + * If this pte is wr-protected by uffd-wp in any form, arm the special pte to
> > > > > > > > + * replace a none pte. NOTE! This should only be called when *pte is already
> > > > > > > > + * cleared so we will never accidentally replace something valuable. Meanwhile
> > > > > > > > + * none pte also means we are not demoting the pte so if tlb flushed then we
> > > > > > > > + * don't need to do it again; otherwise if tlb flush is postponed then it's
> > > > > > > > + * even better.
> > > > > > > > + *
> > > > > > > > + * Must be called with pgtable lock held.
> > > > > > > > + */
> > > > > > > > +static inline void
> > > > > > > > +pte_install_uffd_wp_if_needed(struct vm_area_struct *vma, unsigned long addr,
> > > > > > > > + pte_t *pte, pte_t pteval)
> > > > > > > > +{
> > > > > > > > +#ifdef CONFIG_USERFAULTFD
> > > > > > > > + bool arm_uffd_pte = false;
> > > > > > > > +
> > > > > > > > + /* The current status of the pte should be "cleared" before calling */
> > > > > > > > + WARN_ON_ONCE(!pte_none(*pte));
> > > > > > > > +
> > > > > > > > + if (vma_is_anonymous(vma))
> > > > > > > > + return;
> > > > > > > > +
> > > > > > > > + /* A uffd-wp wr-protected normal pte */
> > > > > > > > + if (unlikely(pte_present(pteval) && pte_uffd_wp(pteval)))
> > > > > > > > + arm_uffd_pte = true;
> > > > > > > > +
> > > > > > > > + /*
> > > > > > > > + * A uffd-wp wr-protected swap pte. Note: this should even work for
> > > > > > > > + * pte_swp_uffd_wp_special() too.
> > > > > > > > + */
> > > > > > >
> > > > > > > I'm probably missing something but when can we actually have this case and why
> > > > > > > would we want to leave a special pte behind? From what I can tell this is
> > > > > > > called from try_to_unmap_one() where this won't be true or from zap_pte_range()
> > > > > > > when not skipping swap pages.
> > > > > >
> > > > > > Yes this is a good question..
> > > > > >
> > > > > > Initially I made this function make sure I cover all forms of uffd-wp bit, that
> > > > > > contains both swap and present ptes; imho that's pretty safe. However for
> > > > > > !anonymous cases we don't keep swap entry inside pte even if swapped out, as
> > > > > > they should reside in shmem page cache indeed. The only missing piece seems to
> > > > > > be the device private entries as you also spotted below.
> > > > >
> > > > > Yes, I think it's *probably* safe although I don't yet have a strong opinion
> > > > > here ...
> > > > >
> > > > > > > > + if (unlikely(is_swap_pte(pteval) && pte_swp_uffd_wp(pteval)))
> > > > >
> > > > > ... however if this can never happen would a WARN_ON() be better? It would also
> > > > > mean you could remove arm_uffd_pte.
> > > >
> > > > Hmm, after a second thought I think we can't make it a WARN_ON_ONCE().. this
> > > > can still be useful for private mapping of shmem files: in that case we'll have
> > > > swap entry stored in pte not page cache, so after page reclaim it will contain
> > > > a valid swap entry, while it's still "!anonymous".

[1]

> > >
> > > There's something (probably obvious) I must still be missing here. During
> > > reclaim won't a private shmem mapping still have a present pteval here?
> > > Therefore it won't trigger this case - the uffd wp bit is set when the swap
> > > entry is established further down in try_to_unmap_one() right?
> >
> > I agree if it's at the point when it get reclaimed, however what if we zap a
> > pte of a page already got reclaimed? It should have the swap pte installed,
> > imho, which will have "is_swap_pte(pteval) && pte_swp_uffd_wp(pteval)"==true.
>
> Apologies for the delay getting back to this, I hope to find some more time
> to look at this again this week.

No problem, please take your time on reviewing the series.

>
> I guess what I am missing is why we care about a swap pte for a reclaimed page
> getting zapped. I thought that would imply the mapping was getting torn down,
> although I suppose in that case you still want the uffd-wp to apply in case a
> new mapping appears there?

For the torn down case it'll always have ZAP_FLAG_DROP_FILE_UFFD_WP set, so
pte_install_uffd_wp_if_needed() won't be called, as zap_drop_file_uffd_wp()
will return true:

static inline void
zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
unsigned long addr, pte_t *pte,
struct zap_details *details, pte_t pteval)
{
if (zap_drop_file_uffd_wp(details))
return;

pte_install_uffd_wp_if_needed(vma, addr, pte, pteval);
}

If you see it's non-trivial to fully digest all the caller stacks of it. What I
wanted to do with pte_install_uffd_wp_if_needed is simply to provide a helper
that can convert any form of uffd-wp ptes into a pte marker before being set as
none pte. Since uffd-wp can exist in two forms (either present, or swap), then
cover all these two forms (and for swap form also cover the uffd-wp special pte
itself) is very clear idea and easy to understand to me. I don't even need to
worry about who is calling it, and which case can be swap pte, which case must
not - we just call it when we want to persist the uffd-wp bit (after a pte got
cleared). That's why in all cases I still prefer to keep it as is, as it just
makes things straightforward to me.

Thanks,

--
Peter Xu