linux-kernel - [PATCH 1/5] mm, swap: Fix comment in __read_swap_cache

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Date:   Fri, 17 Mar 2017 14:46:19 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Shaohua Li <shli@...nel.org>, Rik van Riel <riel@...hat.com>,
        Huang Ying <ying.huang@...el.com>,
        Rafael Aquini <aquini@...hat.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Aaron Lu <aaron.lu@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/5] mm, swap: Fix comment in __read_swap_cache_async

From: Huang Ying <ying.huang@...el.com>

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.

Cc: Shaohua Li <shli@...nel.org>
Cc: Rafael Aquini <aquini@...hat.com>
Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
---
 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