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

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Date:   Thu, 14 Oct 2021 17:57:29 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Rustam Kovhaev <rkovhaev@...il.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.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 Wed, Oct 13, 2021 at 09:56:41AM -0700, Rustam Kovhaev wrote:
> On Wed, Oct 13, 2021 at 09:38:31AM +0200, Vlastimil Babka wrote:
> > On 10/13/21 01:22, Darrick J. Wong wrote:
> > > 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:
> > >> >>
> > >> >> 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.
> > 
> > Agreed. Rustam, do you still plan to do that?
> 
> Yes, I do, thank you.

Note that I left out the parts of the patch that changed mm/slob.c
because I didn't think that was appropriate for a patch titled 'xfs:'.

> 
> > 
> > > 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. :)
> > 
> > Ok. I was just wondering because Dave's first reply was that actually you'll
> > need to expand the use of kfree() instead of kmem_cache_free().

I look forward to doing this, but since XFS is a downstream consumer of
the kmem apis, we'll have to wait until the slob changes land to do
that.

--D