Re: [PATCH 00/12] mm: free retracted page table by RCU

[Hugh Dickins]

On Wed, 31 May 2023, Jann Horn wrote:
> On Mon, May 29, 2023 at 8:11 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > Here is the third series of patches to mm (and a few architectures), based
> > on v6.4-rc3 with the preceding two series applied: in which khugepaged
> > takes advantage of pte_offset_map[_lock]() allowing for pmd transitions.
> 
> To clarify: Part of the design here is that when you look up a user
> page table with pte_offset_map_nolock() or pte_offset_map() without
> holding mmap_lock in write mode, and you later lock the page table
> yourself, you don't know whether you actually have the real page table
> or a detached table that is currently in its RCU grace period, right?

Right.  (And I'd rather not assume anything of mmap_lock, but there are
one or two or three places that may still do so.)

> And detached tables are supposed to consist of only zeroed entries,
> and we assume that no relevant codepath will do anything bad if one of
> these functions spuriously returns a pointer to a page table full of
> zeroed entries?

(Nit that I expect you're well aware of: IIRC "zeroed" isn't 0 on s390.)

If someone is using pte_offset_map() without lock, they must be prepared
to accept page-table-like changes.  The limits of pte_offset_map_nolock()
with later spin_lock(ptl): I'm still exploring: there's certainly an
argument that one ought to do a pmd_same() check before proceeding,
but I don't think anywhere needs that at present.

Whether the page table has to be full of zeroed entries when detached:
I believe it is always like that at present (by the end of the series,
when the collapse_pte_offset_map() oddity is fixed), but whether it needs
to be so I'm not sure.  Quite likely it will need to be; but I'm open to
the possibility that all it needs is to be still a page table, with
perhaps new entries from a new usage in it.

The most obvious vital thing (in the split ptlock case) is that it
remains a struct page with a usable ptl spinlock embedded in it.

The question becomes more urgent when/if extending to replacing the
pagetable pmd by huge pmd in one go, without any mmap_lock: powerpc
wants to deposit the page table for later use even in the shmem/file
case (and all arches in the anon case): I did work out the details once
before, but I'm not sure whether I would still agree with myself; and was
glad to leave replacement out of this series, to revisit some time later.

> 
> So in particular, in handle_pte_fault() we can reach the "if
> (unlikely(!pte_same(*vmf->pte, entry)))" with vmf->pte pointing to a
> detached zeroed page table, but we're okay with that because in that
> case we know that !pte_none(vmf->orig_pte)&&pte_none(*vmf->pte) ,
> which implies !pte_same(*vmf->pte, entry) , which means we'll bail
> out?

There is no current (even at end of series) circumstance in which we
could be pointing to a detached page table there; but yes, I want to
allow for that, and yes I agree with your analysis.  But with the
interesting unanswered question for the future, of what if the same
value could be found there: would that imply it's safe to proceed,
or would some further prevention be needed?

> 
> If that's the intent, it might be good to add some comments, because
> at least to me that's not very obvious.

That's a very fair request; but I shall have difficulty deciding where
to place such comments.  I shall have to try, then you redirect me.

And I think we approach this in opposite ways: my nature is to put some
infrastructure in place, and then look at it to see what we can get away
with; whereas your nature is to define upfront what the possibilities are.
We can expect some tussles!

Thanks,
Hugh