Re: [PATCH 1/5] mm, swap: Fix comment in __read_swap_cache_async

[Rafael Aquini]

On Fri, Mar 17, 2017 at 02:46:19PM +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> The commit cbab0e4eec29 ("swap: avoid read_swap_cache_async() race to
> deadlock while waiting on discard I/O completion") fixed a deadlock in
> read_swap_cache_async().  Because at that time, in swap allocation
> path, a swap entry may be set as SWAP_HAS_CACHE, then wait for
> discarding to complete before the page for the swap entry is added to
> the swap cache.  But in the commit 815c2c543d3a ("swap: make swap
> discard async"), the discarding for swap become asynchronous, waiting
> for discarding to complete will be done before the swap entry is set
> as SWAP_HAS_CACHE.  So the comments in code is incorrect now.  This
> patch fixes the comments.
> 
> The cond_resched() added in the commit cbab0e4eec29 is not necessary
> now too.  But if we added some sleep in swap allocation path in the
> future, there may be some hard to debug/reproduce deadlock bug.  So it
> is kept.
>

^ this is a rather disconcerting way to describe why you left that part
behind, and I recollect telling you about it in a private discussion.

The fact is that __read_swap_cache_async() still races against get_swap_page()
with a way narrower window due to the async fashioned SSD wear leveling 
done for swap nowadays and other changes made within __read_swap_cache_async()'s
while loop thus making that old deadlock scenario very improbable to strike again.

All seems legit, apart from that last paragraph in the commit log
message


Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>
 
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: Rafael Aquini <aquini@xxxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> ---
>  mm/swap_state.c | 12 +-----------
>  1 file changed, 1 insertion(+), 11 deletions(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 473b71e052a8..7bfb9bd1ca21 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -360,17 +360,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  			/*
>  			 * We might race against get_swap_page() and stumble
>  			 * across a SWAP_HAS_CACHE swap_map entry whose page
> -			 * has not been brought into the swapcache yet, while
> -			 * the other end is scheduled away waiting on discard
> -			 * I/O completion at scan_swap_map().
> -			 *
> -			 * In order to avoid turning this transitory state
> -			 * into a permanent loop around this -EEXIST case
> -			 * if !CONFIG_PREEMPT and the I/O completion happens
> -			 * to be waiting on the CPU waitqueue where we are now
> -			 * busy looping, we just conditionally invoke the
> -			 * scheduler here, if there are some more important
> -			 * tasks to run.
> +			 * has not been brought into the swapcache yet.
>  			 */
>  			cond_resched();
>  			continue;
> -- 
> 2.11.0
>