mm/damon/core: set quota->charged_from to jiffies at first charge window

Kernel initializes the "jiffies" timer as 5 minutes below zero, as shown
in include/linux/jiffies.h

 /*
 * Have the 32 bit jiffies value wrap 5 minutes after boot
 * so jiffies wrap bugs show up earlier.
 */
 #define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))

And jiffies comparison help functions cast unsigned value to signed to
cover wraparound

 #define time_after_eq(a,b) \
  (typecheck(unsigned long, a) && \
  typecheck(unsigned long, b) && \
  ((long)((a) - (b)) >= 0))

When quota->charged_from is initialized to 0, time_after_eq() can
incorrectly return FALSE even after reset_interval has elapsed.  This
occurs when (jiffies - reset_interval) produces a value with MSB=1, which
is interpreted as negative in signed arithmetic.

This issue primarily affects 32-bit systems because: On 64-bit systems:
MSB=1 values occur after ~292 million years from boot (assuming HZ=1000),
almost impossible.

On 32-bit systems: MSB=1 values occur during the first 5 minutes after
boot, and the second half of every jiffies wraparound cycle, starting from
day 25 (assuming HZ=1000)

When above unexpected FALSE return from time_after_eq() occurs, the
charging window will not reset.  The user impact depends on esz value at
that time.

If esz is 0, scheme ignores configured quotas and runs without any limits.

If esz is not 0, scheme stops working once the quota is exhausted.  It
remains until the charging window finally resets.

So, change quota->charged_from to jiffies at damos_adjust_quota() when it
is considered as the first charge window.  By this change, we can avoid
unexpected FALSE return from time_after_eq()

Link: https://lkml.kernel.org/r/20250822025057.1740854-1-ekffu200098@gmail.com
Fixes: 2b8a248d5873 ("mm/damon/schemes: implement size quota for schemes application speed control") # 5.16
Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 106ee8b0f2d5f15a18d1fb9c0f462d9b498f99b7..c2e0b469fd43271afd4fe2bc412e80b63d93b565 100644 (file)
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2111,6 +2111,10 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
        if (!quota->ms && !quota->sz && list_empty(&quota->goals))
                return;
 
+       /* First charge window */
+       if (!quota->total_charged_sz && !quota->charged_from)
+               quota->charged_from = jiffies;
+
        /* New charge window starts */
        if (time_after_eq(jiffies, quota->charged_from +
                                msecs_to_jiffies(quota->reset_interval))) {