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

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Date:   Tue, 12 Oct 2021 16:22:55 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     David Rientjes <rientjes@...gle.com>,
        Rustam Kovhaev <rkovhaev@...il.com>,
        Dave Chinner <david@...morbit.com>, linux-xfs@...r.kernel.org,
        cl@...ux.com, penberg@...nel.org, 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 Tue, Oct 12, 2021 at 11:32:25PM +0200, Vlastimil Babka wrote:
> On 10/12/2021 10:43 PM, Darrick J. Wong wrote:
> > On Tue, Oct 12, 2021 at 01:43:20PM -0700, Darrick J. Wong wrote:
> >> On Sun, Oct 03, 2021 at 06:07:20PM -0700, David Rientjes wrote:
> >>> On Thu, 30 Sep 2021, Rustam Kovhaev 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.
> >>>>
> >>>
> >>> It's probably best to introduce both (SLOB fix and Kconfig change for 
> >>> XFS), at least in the interim because the combo of XFS and SLOB could be 
> >>> broken in other ways.  If syzbot doesn't complain with a patched kernel to 
> >>> allow SLOB to be used with XFS, then we could potentially allow them to be 
> >>> used together.
> >>>
> >>> (I'm not sure that this freeing issue is the *only* thing that is broken, 
> >>> nor that we have sufficient information to make that determination right 
> >>> now..)
> >>
> >> I audited the entire xfs (kernel) codebase and didn't find any other
> >> usage errors.  Thanks for the patch; I'll apply it to for-next.
> 
> Which patch, the one that started this thread and uses kmem_cache_free() instead
> of kfree()? I thought we said it's not the best way?

It's probably better to fix slob to be able to tell that a kmem_free'd
object actually belongs to a cache and should get freed that way, just
like its larger sl[ua]b cousins.

However, even if that does come to pass, anybody /else/ who wants to
start(?) using XFS on a SLOB system will need this patch to fix the
minor papercut.  Now that I've checked the rest of the codebase, I don't
find it reasonable to make XFS mutually exclusive with SLOB over two
instances of slab cache misuse.  Hence the RVB. :)

--D

> > Also, the obligatory
> > 
> > Reviewed-by: Darrick J. Wong <djwong@...nel.org>
> > 
> > --D
> > 
> >>
> >> --D
>