linux-kernel - Re: [RFC] [PATCH 0/5] procfs: reduce duplication by using symlinks

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Date:   Wed, 25 Apr 2018 14:04:30 -0400
From:   Jeff Mahoney <jeffm@...e.com>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC] [PATCH 0/5] procfs: reduce duplication by using symlinks

On 4/24/18 2:17 AM, Alexey Dobriyan wrote:
> On Mon, Apr 23, 2018 at 10:21:01PM -0400, jeffm@...e.com wrote:
>> Memory pressure isn't really an issue on this machine, so we
>> end up using well over 100GB for proc files.
> 
> Text files at scale!
> 
>> With these patches applied, running the same testcase, the proc_inode
>> cache only gets to about 600k objects, which is about 99.7% fewer.  I
>> get that procfs isn't supposed to be scalable, but this is kind of
>> extreme. :)>
> Easy stuff:
> * all ->get_link hooks are broken in RCU lookup (use GFP_KERNEL),

It's a pretty common pattern in the kernel, but it's just as easy to set
inode->i_link during instantiation and keep RCU lookup.  There aren't so
many of these to make it a real burden on memory.

> * "%.*s" for dentry names is probably unnecessary,
>   they're always NUL terminated

Ack.

> * kasprintf() does printing twice, since we're kind of care about /proc
>   performance, allocate for the worst case.

Ack, integrated with ->get_link fix.

> * "int nlinks = nlink_tgid;"
>   Unsigned police.

Ack.  nlink_t{,g}id are both u8, but it's easy to make it consistent.

> * (inode->i_mode & S_IFLNK)
> 	this is sketchy, S_ISLNK exists.
> 

Ack.

Notes of my own:

proc_task_count_links also had the logic backward.  It would add an
extra link to the count for the symlink rather than the dir.

proc_pid_files_revalidate only needs to check if the tasks share files
since it won't be called if it's not a symlink.

Thanks for the review,

-Jeff

-- 
Jeff Mahoney
SUSE Labs