Re: [PATCH v2 2/2] mm, memcg: fix memory.peak reset clobbering other fds' watermark

[Ridong Chen <ridong.chen@linux.dev>]

On 8/13/2026 12:48 AM, Johannes Weiner wrote:
> On Fri, Aug 07, 2026 at 05:00:00PM +0800, Ridong wrote:
>> From: Ridong Chen <chenridong@xiaomi.com>
>>
>> Writing to memory.peak resets the peak for that fd only. Each fd is a
>> watcher and reads back max(its own value, the shared local_watermark).
>>
>> peak_write() resets by lowering local_watermark to the current usage.
>> To keep the other watchers' peaks it then walks the watcher list, but it
>> stores the current usage into them instead of the old watermark. So once
>> usage has dropped from a peak, a reset on one fd wrongly drags every
>> other fd's peak down too, even fds that never reset.
>>
>> Reproduced on 7.2.0-rc5-next under QEMU, two fds A and B on one cgroup:
>> B sees the peak (410624 KB), usage drops, then A resets -- and B's peak
>> collapses to 1060 KB although B never reset. With this patch B keeps
>> reading 410624 KB.
>>
>> Fix: save the old watermark before lowering it and use that to floor the
>> other watchers, so a reset only affects the fd that issued it.
>>
>> Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers")
>> Assisted-by: Claude:claude-opus-4-8
>> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
>> ---
>>   mm/memcontrol.c | 7 ++++---
>>   1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 2da55b778ae3..28577beeb3d0 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -4746,7 +4746,7 @@ static ssize_t peak_write(struct kernfs_open_file *of, char *buf, size_t nbytes,
>>   			  loff_t off, struct page_counter *pc,
>>   			  struct list_head *watchers)
>>   {
>> -	unsigned long usage;
>> +	unsigned long usage, peer_watermark;
>>   	struct cgroup_of_peak *peer_ctx;
>>   	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
>>   	struct cgroup_of_peak *ofp = of_peak(of);
>> @@ -4754,11 +4754,12 @@ static ssize_t peak_write(struct kernfs_open_file *of, char *buf, size_t nbytes,
>>   	spin_lock(&memcg->peaks_lock);
>>   
>>   	usage = page_counter_read(pc);
>> +	peer_watermark = max(usage, READ_ONCE(pc->local_watermark));
>>   	WRITE_ONCE(pc->local_watermark, usage);
>>   
>>   	list_for_each_entry(peer_ctx, watchers, list)
>> -		if (usage > peer_ctx->value)
>> -			WRITE_ONCE(peer_ctx->value, usage);
>> +		if (peer_ctx != ofp && peer_watermark > peer_ctx->value)
>> +			WRITE_ONCE(peer_ctx->value, peer_watermark);
> 
> Sorry for letting your previous reply sit unanswered. You made a good
> point on the peer_watermark = max(usage, local_watermark) being
> pointless because that's how local_watermark moves to begin with.
> 
> So what you had before was indeed better. It was just me missing that
> detail. Could you please go back to your original? Feel free to
> include:
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Sure, thank you for your review.

-- 
Best regards
Ridong