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

[David Hildenbrand]

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.


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.

--
Thanks,

David / dhildenb