Re: [PATCH] memcg: Keep socket_pressure fresh on 32-bit kernel.

[Kuniyuki Iwashima <kuniyu@google.com>]

From: Shakeel Butt <shakeel.butt@linux.dev>
Date: Wed, 16 Jul 2025 12:59:13 -0700
> On Wed, Jul 16, 2025 at 04:29:12AM +0000, Kuniyuki Iwashima wrote:
> > memcg->socket_pressure is initialised with jiffies when the memcg
> > is created.
> > 
> > Once vmpressure detects that the cgroup is under memory pressure,
> > the field is updated with jiffies + HZ to signal the fact to the
> > socket layer and suppress memory allocation for one second.
> > 
> > Otherwise, the field is not updated.
> > 
> > mem_cgroup_under_socket_pressure() uses time_before() to check if
> > jiffies is less than memcg->socket_pressure, and this has a bug on
> > 32-bit kernel.
> > 
> >   if (time_before(jiffies, memcg->socket_pressure))
> >           return true;
> > 
> > As time_before() casts the final result to long, the acceptable delta
> > between two timestamps is 2 ^ (BITS_PER_LONG - 1).
> > 
> > On 32-bit kernel with CONFIG_HZ=1000, this is about 24 days.
> > 
> >   >>> (2 ** 31) / 1000 / 60 / 60 / 24
> >   24.855134814814818
> > 
> > Once 24 days have passed since the last update of socket_pressure,
> > mem_cgroup_under_socket_pressure() starts to lie until the next
> > 24 days pass.
> > 
> > Thus, we need to update socket_pressure to a recent timestamp
> > periodically on 32-bit kernel.
> > 
> > Let's do that every 24 hours, with a variation of about 0 to 4 hours.
> > 
> > The variation is to avoid bursting by cgroups created within a small
> > timeframe, like boot-up.
> > 
> > The work could be racy but does not take vmpr->sr_lock nor re-evaluate
> > vmpressure_calc_level() under the assumption that socket_pressure will
> > get updated soon if under memory pressure, because it persists only
> > for one second.
> > 
> > Note that we don't need to worry about 64-bit machines unless they
> > serve for 300 million years.
> > 
> >   >>> (2 ** 63) / 1000 / 60 / 60 / 24 / 365
> >   292471208.6775361
> > 
> > Fixes: 8e8ae645249b8 ("mm: memcontrol: hook up vmpressure to socket pressure")
> > Reported-by: Neal Cardwell <ncardwell@google.com>
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> 
> Is this a real issue which you have seen in the production?

No, this is a theoretical issue that Neal found a while ago.


> I wonder if
> we can just reset memcg->socket_pressure in
> mem_cgroup_under_socket_pressure() for 32 bit systems if we see
> overflow?

I guess you mean like below ?  This doesn't work when jiffies is close
to ULONG_MAX and socket_pressure is close to 0 and the 'actual' time delta
is within HZ.

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 87b6688f124a7..2bf92514b67ff 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1609,6 +1609,10 @@ static inline bool mem_cgroup_under_socket_pressure(struct mem_cgroup *memcg)
 		return !!memcg->tcpmem_pressure;
 #endif /* CONFIG_MEMCG_V1 */
 	do {
+#if BITS_PER_LONG == 32
+		if (jiffies - READ_ONCE(memcg->socket_pressure) > LONG_MAX) {
+			WRITE_ONCE(memcg->socket_pressure, jiffies - HZ);
+#endif
 		if (time_before(jiffies, READ_ONCE(memcg->socket_pressure)))
 			return true;
 	} while ((memcg = parent_mem_cgroup(memcg)));