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mm/damon/sysfs: change next_update_jiffies to a global variable

In DAMON's damon_sysfs_repeat_call_fn(), time_before() is used to compare
the current jiffies with next_update_jiffies to determine whether to
update the sysfs files at this moment.

On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make
jiffies wrap bugs appear earlier. However, this causes time_before() in
damon_sysfs_repeat_call_fn() to unexpectedly return true during the first
5 minutes after boot on 32-bit systems (see [1] for more explanation,
which fixes another jiffies-related issue before). As a result, DAMON
does not update sysfs files during that period.

There is also an issue unrelated to the system's word size[2]: if the
user stops DAMON just after next_update_jiffies is updated and restarts
it after 'refresh_ms' or a longer delay, next_update_jiffies will retain
an older value, causing time_before() to return false and the update to
happen earlier than expected.

Fix these issues by making next_update_jiffies a global variable and
initializing it each time DAMON is started.

Link: https://lkml.kernel.org/r/20251030020746.967174-3-yanquanmin1@huawei.com
Link: https://lkml.kernel.org/r/20250822025057.1740854-1-ekffu200098@gmail.com [1]
Link: https://lore.kernel.org/all/20251029013038.66625-1-sj@kernel.org/ [2]
Fixes: d809a7c64ba8 ("mm/damon/sysfs: implement refresh_ms file internal work")
Suggested-by: SeongJae Park <sj@kernel.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: ze zuo <zuoze1@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

authored by

Quanmin Yan and committed by
Andrew Morton 9fd7bb50 2f6ce7e7

+7 -3
+7 -3
mm/damon/sysfs.c
··· 1552 1552 return ctx; 1553 1553 } 1554 1554 1555 + static unsigned long damon_sysfs_next_update_jiffies; 1556 + 1555 1557 static int damon_sysfs_repeat_call_fn(void *data) 1556 1558 { 1557 1559 struct damon_sysfs_kdamond *sysfs_kdamond = data; 1558 - static unsigned long next_update_jiffies; 1559 1560 1560 1561 if (!sysfs_kdamond->refresh_ms) 1561 1562 return 0; 1562 - if (time_before(jiffies, next_update_jiffies)) 1563 + if (time_before(jiffies, damon_sysfs_next_update_jiffies)) 1563 1564 return 0; 1564 - next_update_jiffies = jiffies + 1565 + damon_sysfs_next_update_jiffies = jiffies + 1565 1566 msecs_to_jiffies(sysfs_kdamond->refresh_ms); 1566 1567 1567 1568 if (!mutex_trylock(&damon_sysfs_lock)) ··· 1607 1606 return err; 1608 1607 } 1609 1608 kdamond->damon_ctx = ctx; 1609 + 1610 + damon_sysfs_next_update_jiffies = 1611 + jiffies + msecs_to_jiffies(kdamond->refresh_ms); 1610 1612 1611 1613 repeat_call_control->fn = damon_sysfs_repeat_call_fn; 1612 1614 repeat_call_control->data = kdamond;