linux-kernel - Fwd: [RFC][PATCH 00/10] On inode::i_count and the usage vs reference count issue

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Message-ID: <CA+55aFzvFraFZVzSstSuw4UzdCAfO29ijV2bVZ+W_750MYYfWQ@mail.gmail.com>
Date:   Fri, 24 Feb 2017 11:24:25 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Hans Liljestrand <ishkamiel@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Windsor <dwindsor@...il.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Chris Mason <clm@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>
Subject: Fwd: [RFC][PATCH 00/10] On inode::i_count and the usage vs reference
 count issue

[ Gaah, resending because I was once again bitten by the html email
problem on android and the list rules.  Sorry for duplicates for the
people who didn't bounce the html ]

On Feb 24, 2017 08:39, "Peter Zijlstra" <peterz@...radead.org> wrote:
>
> To make matters worse; once i_count is a refcount, it appears trivial to avoid
> inode_hash_lock for lookups (yay RCU!) and looking at i_count becomes even more
> of a problem because then holding i_lock will not in fact stabilize it anymore.

Note that if this is the main reason for the series, I would argue
against this all.

Inode lookup is simply not an important function. Inodes just aren't
the primary data structure in the vfs layer, and they are seldom
looked up, since the common operation is too look up the dentry (that
then has a direct pointer to the inode).

Inode lookup tends too happen in places like file creation etc, where
the real costs are elsewhere (ie complex locking cost for directories
and for filesystem inode number generation etc).

So you actually have a load where this all is even noticeable? If I
recall correctly, the loads where we have seen inode locking etc
issues have been things like very high rate socket create/destroy, but
those were generally disable by just avoiding all the inode games
entirely (is not putting the inode on any superblock lists, and never
looking anything up, just always allocating a new inode).

Also note that for the same reason, the usual refcount overflow
advantages are rather questionable. There are very few things that
increment the inode use count, and they aren't generally under direct
user control. Users get references to dentries, not inodes, when they
open or map files. So the inode counts tend to be limited by the
number of hard links you can have to things (or number of mounts you
can do) which are much more limited than the refcount anyway.

So I get the feeling that you should worry about other refcount users
long before you worry about inodes. They may be very central in
traditional UNIX, but not so much in Linux (any more).

                    Linus