Re: [PATCH v1 1/2] mm: let pte_lockptr() consume a pte_t pointer

[David Hildenbrand]

On 29.07.24 18:39, Peter Xu wrote:
> On Mon, Jul 29, 2024 at 12:26:00PM -0400, Peter Xu wrote:
> > On Fri, Jul 26, 2024 at 11:48:01PM +0200, David Hildenbrand wrote:
> > > On 26.07.24 23:28, Peter Xu wrote:
> > > > On Fri, Jul 26, 2024 at 06:02:17PM +0200, David Hildenbrand wrote:
> > > > > On 26.07.24 17:36, Peter Xu wrote:
> > > > > > On Thu, Jul 25, 2024 at 08:39:54PM +0200, David Hildenbrand wrote:
> > > > > > > pte_lockptr() is the only *_lockptr() function that doesn't consume
> > > > > > > what would be expected: it consumes a pmd_t pointer instead of a pte_t
> > > > > > > pointer.
> > > > > > >
> > > > > > > Let's change that. The two callers in pgtable-generic.c are easily
> > > > > > > adjusted. Adjust khugepaged.c:retract_page_tables() to simply do a
> > > > > > > pte_offset_map_nolock() to obtain the lock, even though we won't actually
> > > > > > > be traversing the page table.
> > > > > > >
> > > > > > > This makes the code more similar to the other variants and avoids other
> > > > > > > hacks to make the new pte_lockptr() version happy. pte_lockptr() users
> > > > > > > reside now only in  pgtable-generic.c.
> > > > > > >
> > > > > > > Maybe, using pte_offset_map_nolock() is the right thing to do because
> > > > > > > the PTE table could have been removed in the meantime? At least it sounds
> > > > > > > more future proof if we ever have other means of page table reclaim.
> > > > > >
> > > > > > I think it can't change, because anyone who wants to race against this
> > > > > > should try to take the pmd lock first (which was held already)?
> > > > >
> > > > > That doesn't explain why it is safe for us to assume that after we took the
> > > > > PMD lock that the PMD actually still points at a completely empty page
> > > > > table. Likely it currently works by accident, because we only have a single
> > > > > such user that makes this assumption. It might certainly be a different once
> > > > > we asynchronously reclaim page tables.
> > > >
> > > > I think it's safe because find_pmd_or_thp_or_none() returned SUCCEED, and
> > > > we're holding i_mmap lock for read.  I don't see any way that this pmd can
> > > > become a non-pgtable-page.
> > > >
> > > > I meant, AFAIU tearing down pgtable in whatever sane way will need to at
> > > > least take both mmap write lock and i_mmap write lock (in this case, a file
> > > > mapping), no?
> > >
> > > Skimming over [1] where I still owe a review I think we can now do it now
> > > purely under the read locks, with the PMD lock held.
> >
> > Err, how I missed that.. yeah you're definitely right, and that's the
> > context here where we're collapsing.  I think I somehow forgot all Hugh's
> > work when I replied there, sorry.
> >
> > > I think this is also what collapse_pte_mapped_thp() ends up doing: replace a
> > > PTE table that maps a folio by a PMD (present or none, depends) that maps a
> > > folio only while holding the mmap lock in read mode. Of course, here the
> > > table is not empty but we need similar ways of making PT walkers aware of
> > > concurrent page table retraction.
> > >
> > > IIRC, that was the magic added to __pte_offset_map(), such that
> > > pte_offset_map_nolock/pte_offset_map_lock can fail on races.
> >
> > Said that, I still think current code (before this patch) is safe, same to
> > a hard-coded line to lock the pte pgtable lock.  Again, I'm fine if you
> > prefer pte_offset_map_nolock() but I just think the rcu read lock and stuff
> > can be avoided.
> >
> > I think it's because such collapse so far can only happen in such path
> > where we need to hold the large folio (PMD-level) lock first.  It means
> > anyone who could change this pmd entry to be not a pte pgtable is blocked
> > already, hence it must keeping being a pte pgtable page even if we don't
> > take any rcu.
> >
> > > But if we hold the PMD lock, nothing should actually change (so far my
> > > understanding) -- we cannot suddenly rip out a page table.
> > >
> > > [1]
> > > https://lkml.kernel.org/r/cover.1719570849.git.zhengqi.arch@xxxxxxxxxxxxx
> > >
> > > > > But yes, the PMD cannot get modified while we hold the PMD lock, otherwise
> > > > > we'd be in trouble
> > > > >
> > > > > > I wonder an open coded "ptlock_ptr(page_ptdesc(pmd_page(*pmd)))" would be
> > > > > > nicer here, but only if my understanding is correct.
> > > > >
> > > > > I really don't like open-coding that. Fortunately we were able to limit the
> > > > > use of ptlock_ptr to a single user outside of arch/x86/xen/mmu_pv.c so far.
> > > >
> > > > I'm fine if you prefer like that; I don't see it a huge deal to me.
> > >
> > > Let's keep it like that, unless we can come up with something neater. At
> > > least it makes the code also more consistent with similar code in that file
> > > and the overhead should be  minimal.
> > >
> > > I was briefly thinking about actually testing if the PT is full of
> > > pte_none(), either as a debugging check or to also handle what is currently
> > > handled via:
> > >
> > > if (likely(!vma->anon_vma && !userfaultfd_wp(vma))) {
> > >
> > > Seems wasteful just because some part of a VMA might have a private page
> > > mapped / uffd-wp active to let all other parts suffer.
> > >
> > > Will think about if that is really worth it.
> > >
> > > ... also because I still want to understand why the PTL of the PMD table is
> > > required at all. What if we lock it first and somebody else wants to lock it
> > > after us while we already ripped it out? Sure there must be some reason for
> > > the lock, I just don't understand it yet :/.
> >
> > IIUC the pte pgtable lock will be needed for checking anon_vma safely.
> >
> > e.g., consider if we don't take the pte pgtable lock, I think it's possible
> > some thread tries to inject a private pte (and prepare anon_vma before
> > that) concurrently with this thread trying to collapse the pgtable into a
> > huge pmd.  I mean, when without the pte pgtable lock held, I think it's
> > racy to check this line:
> >
> >          if (unlikely(vma->anon_vma || userfaultfd_wp(vma))) {
> >                  ...
> >          }
> >
> > On the 1st condition.
>
> Hmm, right after I replied, I think it also guarantees safety on the 2nd
> condition..
>
> Note that one thing I still prefer a hard-coded line over
> pte_offset_map_nolock() is that, the new code seems to say we can treat the
> pte pgtable page differently from the pte pgtable lock.  But I think
> they're really in the same realm.
>
> In short, AFAIU the rcu lock not only protects the pte pgtable's existance,
> but also protects the pte lock.
>
>  From that POV, below new code in this patch:
>
> -               ptl = pte_lockptr(mm, pmd);
> +
> +               /*
> +                * No need to check the PTE table content, but we'll grab the
> +                * PTE table lock while we zap it.
> +                */
> +               pte = pte_offset_map_nolock(mm, pmd, addr, &ptl);
> +               if (!pte)
> +                       goto unlock_pmd;
>                  if (ptl != pml)
>                          spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
> +               pte_unmap(pte);
>
> Could be very misleading, where it seems to say that it's fine to use the
> pte pgtable lock even after rcu unlock.  It might make the code harder to
> understand.

I see what you mean but this is a very similar pattern as used in 
collapse_pte_mapped_thp(), no? There we have

start_pte = pte_offset_map_nolock(mm, pmd, haddr, &ptl);
...
if (!pml)
	spin_lock(ptl);
...
pte_unmap(start_pte);
if (!pml)
	spin_unlock(ptl);


Again, I don't have a strong opinion on this, but doing it more similar 
to collapse_pte_mapped_thp() to obtain locks makes it clearer to me. But 
if I am missing something obvious please shout and I'll change it.

--
Cheers,

David / dhildenb