Re: [PATCH v2] memcg: Convert memcg->socket_pressure to u64.

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jul 17, 2025 at 07:46:43PM +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.
> 
> We don't need to worry about this on 64-bit machines unless they
> serve for 300 million years.
> 
>   >>> (2 ** 63) / 1000 / 60 / 60 / 24 / 365
>   292471208.6775361
> 
> Let's convert memcg->socket_pressure to u64.
> 
> Fixes: 8e8ae645249b8 ("mm: memcontrol: hook up vmpressure to socket pressure")
> Reported-by: Neal Cardwell <ncardwell@google.com>
> Suggested-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> ---
> v2: No delayed work, simply make socket_pressure u64
> 
> v1: https://lore.kernel.org/all/20250716042925.106239-1-kuniyu@google.com/T/#u
> ---

Acked-by: Johannes Weiner <hannes@cmpxchg.org>