From: Huang Ying <ying.huang@xxxxxxxxx>
Subject: mm, swap: fix comment in __read_swap_cache_async
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 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.
Link: http://lkml.kernel.org/r/20170317064635.12792-1-ying.huang@xxxxxxxxx
Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>
Cc: Shaohua Li <shli@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/swap_state.c | 12 +-----------
1 file changed, 1 insertion(+), 11 deletions(-)
diff -puN mm/swap_state.c~mm-swap-fix-comment-in-__read_swap_cache_async mm/swap_state.c
--- a/mm/swap_state.c~mm-swap-fix-comment-in-__read_swap_cache_async
+++ a/mm/swap_state.c
@@ -360,17 +360,7 @@ struct page *__read_swap_cache_async(swp
/*
* 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;
_
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