linux-kernel - [RFC PATCH] mm/swapfile: add cond_resched() in get_swap

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Date:   Sat, 28 Jan 2023 07:20:14 +0000
From:   Longlong Xia <xialonglong1@...wei.com>
To:     <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <ying.huang@...el.com>, <chenwandun@...wei.com>,
        <wangkefeng.wang@...wei.com>, <sunnanyong@...wei.com>,
        Longlong Xia <xialonglong1@...wei.com>
Subject: [RFC PATCH] mm/swapfile: add cond_resched() in get_swap_pages()

We found that the plist_for_each_entry_safe() loop in get_swap_pages()
could reach tens of thousands of times to find available space (extreme
case: cond_resched() is not called in the loop). The test scenario is like
commit de1ccfb64824 ("swapfile: fix soft lockup in scan_swap_map_slots")
does. On the other hand, scan_swap_map_slots() needs to reach LATENCY_LIMIT
before calling cond_resched(), so cond_resched() is needed when failed to
find available space to avoid softlockup.

Signed-off-by: Longlong Xia <xialonglong1@...wei.com>
---
 mm/swapfile.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 908a529bca12..4fa440e87cd6 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1100,6 +1100,7 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
 			goto check_out;
 		pr_debug("scan_swap_map of si %d failed to find offset\n",
 			si->type);
+		cond_resched();

 		spin_lock(&swap_avail_lock);
 nextsi:
-- 
2.25.1