Re: [QUESTION] anon_vma lock in khugepaged

[David Hildenbrand]

On 05.12.24 11:10, Dev Jain wrote:
> On 28/11/24 11:56 am, Dev Jain wrote:
> > Hi, I was looking at khugepaged code and I cannot figure out what will the problem be
> > if we take the mmap lock in read mode. Shouldn't just taking the PMD lock, then PTL,
> > then unlocking PTL, then unlocking PMD, solve any races with page table walkers?
>
> Similar questions:
>
> 1. Why do we need anon_vma_lock_write() in collapse_huge_page()? AFAIK we need to walk anon_vma's either
>      when we are forking or when we are unmapping a folio and need to find all VMAs mapping it; the former path takes the
>      mmap_write_lock() and so we have no problem, and for the latter, if we just had anon_vma_lock_read(), then it
>      may happen that kswapd isolates folio from LRU, and traverses rmap and swaps the folio out and khugepaged fails in
>      folio_isolate_lru(), but then that is not a fatal problem but just a performance degradation due to a race (wherein
>      the entire code is racy anyways). What am I missing?
>
> 2. In what all scenarios does rmap come into play? Fork, swapping out, any other I am missing?
>
> 3. Please confirm the correctness: In stark contrast to page migration, we do not need to do rmap walk and nuke all
>      PTEs referencing the folio, because for anon non-shmem folios, the only way the folio can be shared is forking,
>      and, if that is the case, folio_put() will not release the folio in __collapse_huge_page_copy_succeeded() -> free_page_and_swap_cache(),
>      so the old folio is still there and child processes can read from it. Page migration requires that we are able
>      to deallocate the old folios.

Without going too much into detail: I think one reason is that 
try_to_unmap() and friends should not fail (not finding all PTEs/PMDs) 
simply because there is concurrent collapse going on.

Observe how try_to_unmap() and friends take the folio lock first, and 
how collapse tries taking the folio locks only after it already 
invalidated the PMDP.

Likely there are other reasons, but this is the "obvious" one I can 
think of.

We really try preventing *any* concurrent page table walkers, and that 
requires the mmap lock+vma lock in write + rmap lock in write.

--
Cheers,

David / dhildenb