linux-kernel - Re: [PATCH] testing/radix-tree/maple: hack around kfree

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Message-ID: <fcbr3lvd2aa6m4cjl666ksbf5px25htnh5slahj4pk2id54ygn@llqqfn5urq52>
Date: Fri, 15 Aug 2025 13:34:02 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Matthew Wilcox <willy@...radead.org>, Sidhartha Kumar <sidhartha.kumar@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, maple-tree@...ts.infradead.org, linux-mm@...ck.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] testing/radix-tree/maple: hack around kfree_rcu not
 existing

On Thu, Aug 14, 2025 at 10:09:15PM -0400, Liam R. Howlett wrote:
> * Andrew Morton <akpm@...ux-foundation.org> [250814 21:02]:
> > On Thu, 14 Aug 2025 13:40:03 +0100 Pedro Falcato <pfalcato@...e.de> wrote:
> > 
> > > On Thu, Aug 14, 2025 at 07:49:27AM +0100, Lorenzo Stoakes wrote:
> > > > From: Pedro Falcato <pfalcato@...e.de>
> > > > 
> > > > liburcu doesn't have kfree_rcu (or anything similar). Despite that, we can
> > > > hack around it in a trivial fashion, by adding a wrapper.
> > > > 
> > > > This wrapper only works for maple_nodes, and not anything else (due to us
> > > > not being able to know rcu_head offsets in any way), and thus we take
> > > > advantage of the type checking to avoid future silent breakage.
> > > > 
> > > > This fixes the build for the VMA userland tests.
> > > > 
> > > > Additionally remove the existing implementation in maple.c, and have
> > > > maple.c include the maple-shared.c header.
> > > > 
> > > > Reviewed-by: Sidhartha Kumar <sidhartha.kumar@...cle.com>
> > > > Tested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > > > Signed-off-by: Pedro Falcato <pfalcato@...e.de>
> > > > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > > > ---
> > > > 
> > > > Andrew - please attribute this as Pedro's patch (Pedro - please mail to
> > > > confirm), as this is simply an updated version of [0], pulled out to fix the
> > > > VMA tests which remain broken.
> > > >
> > > 
> > > ACK, this is fine. The future of the series is still unclear, so if this fixes
> > > the build then all good from my end :)
> > 
> > Well, can we have this as a standalone thing, rather than as a
> > modification to a patch whose future is uncertain?
> > 
> > Then we can just drop "testing/radix-tree/maple: hack around kfree_rcu
> > not existing", yes?
> > 
> > Some expansion of "fixes the build for the VMA userland tests" would be
> > helpful.
> 
> Ah, this is somewhat messy.
> 
> Pedro removed unnecessary rcu calls with the newer slab reality as you
> can directly call kfree instead of specifying the kmem_cache.
> 
> But the patch is partially already in Vlastimil's sheaves work and we'd
> like his work to go through his branch, so the future of this particular
> patch is a bit messy.
> 
> Maybe we should just drop the related patches that caused the issue from
> the mm-new branch?  That way we don't need a fix at all.
> 
> And when Vlastimil is around, we can get him to pick up the set
> including the fix.
> 
> Doing things this way will allow Vlastimil the avoid conflicts on
> rebase, and restore the userspace testing in mm-new.
> 
> Does that make sense to everyone?
>

I agree. This sounds sensible. I don't think it makes much sense to let the
patchset rot in mm-new.

-- 
Pedro