linux-kernel - Re: [PATCH v4 00/10] tracing: introducing eventfs

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Message-ID: <66DACCE8-1C57-4A30-8ACF-D88179CA6381@vmware.com>
Date:   Fri, 21 Jul 2023 17:17:24 +0000
From:   Nadav Amit <namit@...are.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Ajay Kaher <akaher@...are.com>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        Ching-lin Yu <chinglinyu@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-trace-kernel@...r.kernel.org" 
        <linux-trace-kernel@...r.kernel.org>,
        kernel test robot <lkp@...el.com>,
        "oe-lkp@...ts.linux.dev" <oe-lkp@...ts.linux.dev>,
        Alexey Makhalov <amakhalov@...are.com>,
        "er.ajay.kaher@...il.com" <er.ajay.kaher@...il.com>,
        "srivatsa@...il.mit.edu" <srivatsa@...il.mit.edu>,
        Tapas Kundu <tkundu@...are.com>,
        Vasavi Sirnapalli <vsirnapalli@...are.com>
Subject: Re: [PATCH v4 00/10] tracing: introducing eventfs


> On Jul 21, 2023, at 6:19 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
>>      union {
>> +             struct list_head        del_list;
>>              struct rcu_head         rcu;
>> -             struct llist_node       llist;  /* For freeing after RCU */
>> +             unsigned long           is_freed; /* Freed if one of the above is set */
> 
> I changed the freeing around. The dentries are freed before returning from
> eventfs_remove_dir().
> 
> I also added a "is_freed" field that is part of the union and is set if
> list elements have content. Note, since the union was criticized before, I
> will state the entire purpose of doing this patch set is to save memory.
> This structure will be used for every event file. What's the point of
> getting rid of dentries if we are replacing it with something just as big?
> Anyway, struct dentry does the exact same thing!

Hey, don’t shoot me…

[And admittedly, I didn’t review the whole series after v1.]

I understand your position, but I think that at least is_freed should not
be in the union, and you can just put it after umode_t.

Even for the matter of size, it should not matter in most architectures
since umode_t is 16-bit, as natural alignment is at least 32-bits.

[ And “bool" is clearer type for is_freed. ]