linux-kernel - Re: [PATCH] fs: inode: Reduce volatile inode wraparound risk when ino

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Message-ID: <20191220164632.GA26902@bombadil.infradead.org>
Date:   Fri, 20 Dec 2019 08:46:32 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Chris Down <chris@...isdown.name>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, kernel-team@...com,
        Hugh Dickins <hughd@...gle.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        "zhengbin (A)" <zhengbin13@...wei.com>
Subject: Re: [PATCH] fs: inode: Reduce volatile inode wraparound risk when
 ino_t is 64 bit

On Fri, Dec 20, 2019 at 03:41:11PM +0200, Amir Goldstein wrote:
> Suggestion:
> 1. Extend the kmem_cache API to let the ctor() know if it is
> initializing an object
>     for the first time (new page) or recycling an object.

Uh, what?  The ctor is _only_ called when new pages are allocated.
Part of the contract with the slab user is that objects are returned to
the slab in an initialised state.

> 2. Let shmem_init_inode retain the value of i_ino of recycled shmem_inode_info
>     objects
> 3. i_ino is initialized with get_next_ino() only in case it it zero
> 
> Alternatively to 1., if simpler to implement and acceptable by slab developers:
> 1.b. remove the assertion from cache_grow_begin()/new_slab_objects():
>        WARN_ON_ONCE(s->ctor && (flags & __GFP_ZERO));
>        and pass __GFP_ZERO in shmem_alloc_inode()

WTF would that _mean_?  I want this object to contain zeroes and whatever
the constructor sets it to.  WHich one wins?