linux-kernel - Re: [PATCH] xfs: use kmem_cache_free() for kmem

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Date:   Thu, 30 Sep 2021 17:32:23 -0700
From:   Rustam Kovhaev <rkovhaev@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Dave Chinner <david@...morbit.com>
Cc:     djwong@...nel.org, linux-xfs@...r.kernel.org, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, gregkh@...uxfoundation.org,
        Al Viro <viro@...iv.linux.org.uk>, dvyukov@...gle.com
Subject: Re: [PATCH] xfs: use kmem_cache_free() for kmem_cache objects

On Thu, Sep 30, 2021 at 11:10:10PM +0200, Vlastimil Babka wrote:
> On 9/30/21 8:48 PM, Rustam Kovhaev wrote:
> > On Thu, Sep 30, 2021 at 10:13:40AM +0200, Vlastimil Babka wrote:
> >>
> >> I think it's fair if something like XFS (not meant for tiny systems AFAIK?)
> >> excludes SLOB (meant for tiny systems). Clearly nobody tried to use these
> >> two together last 5 years anyway.
> > 
> > +1 for adding Kconfig option, it seems like some things are not meant to
> > be together.
> 
> But if we patch SLOB, we won't need it.

OK, so we consider XFS on SLOB a supported configuration that might be
used and should be tested.
I'll look into maybe adding a config with CONFIG_SLOB and CONFIG_XFS_FS
to syzbot.

It seems that we need to patch SLOB anyway, because any other code can
hit the very same issue.

> >> Maybe we could also just add the 4 bytes to all SLOB objects, declare
> >> kfree() is always fine and be done with it. Yes, it will make SLOB footprint
> >> somewhat less tiny, but even whan we added kmalloc power of two alignment
> >> guarantees, the impact on SLOB was negligible.
> > 
> > I'll send a patch to add a 4-byte header for kmem_cache_alloc()
> > allocations.
> 
> Thanks. Please report in the changelog slab usage from /proc/meminfo
> before and after patch (at least a snapshot after a full boot).

OK.