linux-kernel - Re: [PATCH] mm: mmap_lock: optimize mmap

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Message-ID: <c3eb6f05-f857-4c3d-9ef8-2488baecb00c@suse.cz>
Date: Sat, 23 Nov 2024 22:38:59 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Shakeel Butt <shakeel.butt@...ux.dev>,
 Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>,
 Axel Rasmussen <axelrasmussen@...gle.com>,
 Steven Rostedt <rostedt@...dmis.org>, Suren Baghdasaryan
 <surenb@...gle.com>, linux-mm@...ck.org, cgroups@...r.kernel.org,
 linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] mm: mmap_lock: optimize mmap_lock tracepoints

On 11/23/24 22:35, Shakeel Butt wrote:
> On Sat, Nov 23, 2024 at 05:01:57PM +0000, Matthew Wilcox wrote:
>> On Fri, Nov 22, 2024 at 10:09:39PM -0800, Shakeel Butt wrote:
>> >  	TP_printk(
>> > -		"mm=%p memcg_path=%s write=%s",
>> > -		__entry->mm,
>> > -		__get_str(memcg_path),
>> > +		"mm=%p memcg_id=%llu write=%s",
>> > +		__entry->mm, __entry->memcg_id,
>> >  		__entry->write ? "true" : "false"
>> 
>> Is it actually useful to print out the (hashed) pointer of the mm?
>> Wouldn't the PID be more useful so you could actually associate it with
>> a task?
>> 
> 
> For our usecase i.e. bpftrace, we don't really care about these prints
> as we can directly access the arguments like mm in bpftrace. I wonder if
> others are using this hased pointer in some other way. I don't mind
> chaning it but I think that would be a separate patch.

I wonder if it's actually hashed when trace events are obtained in binary
form, i.e. via trace-cmd. Might be hashed only when doing e.g. cat
trace_pipe as that's when the kernel's printk with its hashing is used?

I guess that would be another argument for not using it in the tracepoint,
as it would be a sidechannel...