Re: [PATCH] mm: zswap: fix data loss on SWP_SYNCHRONOUS_IO devices

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2024/3/25 05:04, Johannes Weiner wrote:
> Zhongkun He reports data corruption when combining zswap with zram.
> 
> The issue is the exclusive loads we're doing in zswap. They assume
> that all reads are going into the swapcache, which can assume
> authoritative ownership of the data and so the zswap copy can go.
> 
> However, zram files are marked SWP_SYNCHRONOUS_IO, and faults will try
> to bypass the swapcache. This results in an optimistic read of the
> swap data into a page that will be dismissed if the fault fails due to
> races. In this case, zswap mustn't drop its authoritative copy.
> 
> Link: https://lore.kernel.org/all/CACSyD1N+dUvsu8=zV9P691B9bVq33erwOXNTmEaUbi9DrDeJzw@mail.gmail.com/
> Reported-by: Zhongkun He <hezhongkun.hzk@bytedance.com>
> Fixes: b9c91c43412f ("mm: zswap: support exclusive loads")
> Cc: stable@vger.kernel.org	[6.5+]
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Tested-by: Zhongkun He <hezhongkun.hzk@bytedance.com>

Very nice solution!

Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

Thanks.

> ---
>  mm/zswap.c | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 535c907345e0..41a1170f7cfe 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1622,6 +1622,7 @@ bool zswap_load(struct folio *folio)
>  	swp_entry_t swp = folio->swap;
>  	pgoff_t offset = swp_offset(swp);
>  	struct page *page = &folio->page;
> +	bool swapcache = folio_test_swapcache(folio);
>  	struct zswap_tree *tree = swap_zswap_tree(swp);
>  	struct zswap_entry *entry;
>  	u8 *dst;
> @@ -1634,7 +1635,20 @@ bool zswap_load(struct folio *folio)
>  		spin_unlock(&tree->lock);
>  		return false;
>  	}
> -	zswap_rb_erase(&tree->rbroot, entry);
> +	/*
> +	 * When reading into the swapcache, invalidate our entry. The
> +	 * swapcache can be the authoritative owner of the page and
> +	 * its mappings, and the pressure that results from having two
> +	 * in-memory copies outweighs any benefits of caching the
> +	 * compression work.
> +	 *
> +	 * (Most swapins go through the swapcache. The notable
> +	 * exception is the singleton fault on SWP_SYNCHRONOUS_IO
> +	 * files, which reads into a private page and may free it if
> +	 * the fault fails. We remain the primary owner of the entry.)
> +	 */
> +	if (swapcache)
> +		zswap_rb_erase(&tree->rbroot, entry);
>  	spin_unlock(&tree->lock);
>  
>  	if (entry->length)
> @@ -1649,9 +1663,10 @@ bool zswap_load(struct folio *folio)
>  	if (entry->objcg)
>  		count_objcg_event(entry->objcg, ZSWPIN);
>  
> -	zswap_entry_free(entry);
> -
> -	folio_mark_dirty(folio);
> +	if (swapcache) {
> +		zswap_entry_free(entry);
> +		folio_mark_dirty(folio);
> +	}
>  
>  	return true;
>  }