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

[Peter Xu]

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?

> 
> 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.

Thanks,

-- 
Peter Xu