Re: [PATCH] mm: zswap: add VM_BUG_ON() if large folio swapin is attempted

[David Hildenbrand <david@redhat.com>]

On 06.06.24 20:48, Yosry Ahmed wrote:
> With ongoing work to support large folio swapin, it is important to make
> sure we do not pass large folios to zswap_load() without implementing
> proper support.
> 
> For example, if a swapin fault observes that contiguous PTEs are
> pointing to contiguous swap entries and tries to swap them in as a large
> folio, swap_read_folio() will pass in a large folio to zswap_load(), but
> zswap_load() will only effectively load the first page in the folio. If
> the first page is not in zswap, the folio will be read from disk, even
> though other pages may be in zswap.
> 
> In both cases, this will lead to silent data corruption.
> 
> Proper large folio swapin support needs to go into zswap before zswap
> can be enabled in a system that supports large folio swapin.
> 
> Looking at callers of swap_read_folio(), it seems like they are either
> allocated from __read_swap_cache_async() or do_swap_page() in the
> SWP_SYNCHRONOUS_IO path. Both of which allocate order-0 folios, so we
> are fine for now.
> 
> Add a VM_BUG_ON() in zswap_load() to make sure that we detect changes in
> the order of those allocations without proper handling of zswap.
> 
> Alternatively, swap_read_folio() (or its callers) can be updated to have
> a fallback mechanism that splits large folios or reads subpages
> separately. Similar logic may be needed anyway in case part of a large
> folio is already in the swapcache and the rest of it is swapped out.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
> 
> Sorry for the long CC list, I just found myself repeatedly looking at
> new series that add swap support for mTHPs / large folios, making sure
> they do not break with zswap or make incorrect assumptions. This debug
> check should give us some peace of mind. Hopefully this patch will also
> raise awareness among people who are working on this.
> 
> ---
>   mm/zswap.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index b9b35ef86d9be..6007252429bb2 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1577,6 +1577,9 @@ bool zswap_load(struct folio *folio)
>   	if (!entry)
>   		return false;
>   
> +	/* Zswap loads do not handle large folio swapins correctly yet */
> +	VM_BUG_ON(folio_test_large(folio));
> +

There is no way we could have a WARN_ON_ONCE() and recover, right?

-- 
Cheers,

David / dhildenb