linux-kernel - [PATCH] zsmalloc: do not use bit_spin

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Date:   Sun, 20 Dec 2020 02:22:28 +0200
From:   Vitaly Wool <vitaly.wool@...sulko.com>
To:     Mike Galbraith <efault@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Cc:     Barry Song <song.bao.hua@...ilicon.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Minchan Kim <minchan@...nel.org>,
        NitinGupta <ngupta@...are.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>
Subject: [PATCH] zsmalloc: do not use bit_spin_lock

zsmalloc takes bit spinlock in its _map() callback and releases it
only in unmap() which is unsafe and leads to zswap complaining
about scheduling in atomic context.

To fix that and to improve RT properties of zsmalloc, remove that
bit spinlock completely and use a bit flag instead.

Signed-off-by: Vitaly Wool <vitaly.wool@...sulko.com>
---
 mm/zsmalloc.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 7289f502ffac..ff26546a7fed 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -876,22 +876,25 @@ static unsigned long obj_to_head(struct page *page, void *obj)
 
 static inline int testpin_tag(unsigned long handle)
 {
-	return bit_spin_is_locked(HANDLE_PIN_BIT, (unsigned long *)handle);
+	return test_bit(HANDLE_PIN_BIT, (unsigned long *)handle);
 }
 
 static inline int trypin_tag(unsigned long handle)
 {
-	return bit_spin_trylock(HANDLE_PIN_BIT, (unsigned long *)handle);
+	return !test_and_set_bit(HANDLE_PIN_BIT, (unsigned long *)handle);
 }
 
-static void pin_tag(unsigned long handle) __acquires(bitlock)
+static void pin_tag(unsigned long handle)
 {
-	bit_spin_lock(HANDLE_PIN_BIT, (unsigned long *)handle);
+	preempt_disable();
+	while(test_and_set_bit(HANDLE_PIN_BIT, (unsigned long *)handle))
+		cpu_relax();
+	preempt_enable();
 }
 
 static void unpin_tag(unsigned long handle) __releases(bitlock)
 {
-	bit_spin_unlock(HANDLE_PIN_BIT, (unsigned long *)handle);
+	clear_bit(HANDLE_PIN_BIT, (unsigned long *)handle);
 }
 
 static void reset_page(struct page *page)
-- 
2.20.1