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

[Peter Xu <peterx@redhat.com>]

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@bytedance.com
> 
> > 
> > > 
> > > 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.

Thanks,

-- 
Peter Xu