Re: [PATCH 1/6] mm: enable page walking API to lock vmas during the walk

[Peter Xu <peterx@redhat.com>]

On Tue, Aug 01, 2023 at 01:28:56PM -0700, Suren Baghdasaryan wrote:
> I have the new patchset ready but I see 3 places where we walk the
> pages after mmap_write_lock() while *I think* we can tolerate
> concurrent page faults (don't need to lock the vmas):
> 
> s390_enable_sie()
> break_ksm()
> clear_refs_write()

This one doesn't look right to be listed - tlb flushing is postponed after
pgtable lock released, so I assume the same issue can happen like fork():
where we can have race coditions to corrupt data if, e.g., thread A
writting with a writable (unflushed) tlb, alongside with thread B CoWing.

It'll indeed be nice to know whether break_ksm() can avoid that lock_vma
parameter across quite a few function jumps. I don't yet see an immediate
issue with this one..  No idea on s390_enable_sie(), but to make it simple
and safe I'd simply leave it with the write vma lock to match the mmap
write lock.

Thanks,

-- 
Peter Xu