Re: [PATCH v5 4/9] mm: filemap: use xa_get_order() to get the swap entry order

[Matthew Wilcox <willy@infradead.org>]

On Sun, Aug 25, 2024 at 02:55:41PM -0700, Hugh Dickins wrote:
> The second issue is that swap is more slippery to work with than
> folios or pages: in the folio_nr_pages() case, that number is stable
> because we hold a refcount (which stops a THP from being split), and
> later we'll be taking folio lock too.  None of that in the swap case,
> so (depending on how a large entry gets split) the xa_get_order() result
> is not reliable. Likewise for other uses of xa_get_order() in this series.
> 
> There needs to be some kind of locking or retry to make the order usable,
> and to avoid shmem_free_swap() occasionally freeing more than it ought.
> I'll give it thought after.

My original thought was that we'd take a bit from the swap entry in
order to indicate the order of the entry.  I was surprised to see the
xa_get_order() implementation, but didn't remember why it wouldn't work.
Sorry.

Anyway, that's how I think it should be fixed.  Is that enough?  Holding
a reference on the folio prevents truncation, splitting, and so on.
There's no reference to be held on a swap entry, so could we have some
moderately implausible series of operations while holding only the RCU
read lock that would cause us to go wrong?