Re: [PATCH v3 1/3] mm/khugepaged: Take the right locks for page table retraction

[David Hildenbrand]

On 28.11.22 18:28, Jann Horn wrote:
> On Mon, Nov 28, 2022 at 2:53 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 25.11.22 22:37, Jann Horn wrote:
> > > pagetable walks on address ranges mapped by VMAs can be done under the mmap
> > > lock, the lock of an anon_vma attached to the VMA, or the lock of the VMA's
> > > address_space. Only one of these needs to be held, and it does not need to
> > > be held in exclusive mode.
> > >
> > > Under those circumstances, the rules for concurrent access to page table
> > > entries are:
> > >
> > >    - Terminal page table entries (entries that don't point to another page
> > >      table) can be arbitrarily changed under the page table lock, with the
> > >      exception that they always need to be consistent for
> > >      hardware page table walks and lockless_pages_from_mm().
> > >      This includes that they can be changed into non-terminal entries.
> > >    - Non-terminal page table entries (which point to another page table)
> > >      can not be modified; readers are allowed to READ_ONCE() an entry, verify
> > >      that it is non-terminal, and then assume that its value will stay as-is.
> > >
> > > Retracting a page table involves modifying a non-terminal entry, so
> > > page-table-level locks are insufficient to protect against concurrent
> > > page table traversal; it requires taking all the higher-level locks under
> > > which it is possible to start a page walk in the relevant range in
> > > exclusive mode.
> > >
> > > The collapse_huge_page() path for anonymous THP already follows this rule,
> > > but the shmem/file THP path was getting it wrong, making it possible for
> > > concurrent rmap-based operations to cause corruption.
> >
> > This sounds sane and correct to me. No expert on file-THP, though.
> >
> > For anon-THP it's the mmap lock and the rmap locks. I assume the only
> > difference for file-THP is that the rmap lock is actually the mapping
> > lock. Looking at rmap_walk_file(), that seems to be the case.
>
> Yeah. You can also have private file VMAs that are associated with
> both a mapping and a set of anon_vmas, and in that case you would need
> to lock the mmap, the mapping, and the anon_vma root; but the file THP
> code in khugepaged instead just bails on file VMAs with an anon_vma.

Right, that's my understanding as well.

> > I wish at least PTE table removal could be done easier ... I already
> > experimented some time ago with some ideas (e.g., lock in PMD table
> > memmap) but it's all far from trivial and space in the memmap is rare.
>
> Because you want it to be faster? Is that for the THP usecase or something else?

Page table reclaim and page table migration, where you might only have 
limited context and wouldn't want to take all these expensive locks in 
write mode (IOW, you wouldn't want to care about them at all).

Feel free to add my

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb