Re: [PATCH v3 21/30] maple_tree: Add cp_is_new_root() helper

[SeongJae Park]

On Mon, 2 Feb 2026 12:01:53 -0500 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote:

> * SeongJae Park <sj@xxxxxxxxxx> [260202 10:56]:
> > On Mon, 2 Feb 2026 09:58:31 -0500 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote:
> > 
> > > * SeongJae Park <sj@xxxxxxxxxx> [260131 19:10]:
> > > > Hello,
> > > > 
> > > > On Fri, 30 Jan 2026 15:59:26 -0500 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> wrote:
> > > > 
> > > > > Add a helper to do what is needed when the maple copy node contains a
> > > > > new root node.  This is useful for future commits and is
> > > > > self-documenting code.
> > > > > 
> > > > > Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> > > > > ---
> > > > >  lib/maple_tree.c | 70 ++++++++++++++++++++++++++----------------------
> > > > >  1 file changed, 38 insertions(+), 32 deletions(-)
> > > > > 
> > > > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > > > > index 5280fa6d2d6ec..42038e42a4c7e 100644
> > > > > --- a/lib/maple_tree.c
> > > > > +++ b/lib/maple_tree.c
> > > > > @@ -3337,6 +3337,43 @@ static void mas_spanning_rebalance(struct ma_state *mas,
> > > > >  	mas_spanning_rebalance_loop(mas, mast, count);
> > > > >  }
> > > > >  
> > > > > +static inline bool cp_is_new_root(struct maple_copy *cp, struct ma_state *mas)
> > > > > +{
> > > > > +	if (cp->min || cp->max != ULONG_MAX)
> > > > > +		return false;
> > > > > +
> > > > > +	if (cp->d_count != 1) {
> > > > > +		enum maple_type mt = maple_arange_64;
> > > > > +
> > > > > +		if (!mt_is_alloc(mas->tree))
> > > > > +			mt = maple_range_64;
> > > > > +
> > > > > +		cp->data = cp->d_count;
> > > > > +		cp->s_count = 0;
> > > > > +		dst_setup(cp, mas, mt);
> > > > > +		init_cp_src(cp);
> > > > > +		node_copy(mas, cp->src[0].node, 0, cp->data, cp->max, maple_copy,
> > > > > +			  cp->dst[0].node, 0, mt);
> > > > > +		node_finalise(cp->dst[0].node, mt, cp->end + 1);
> > > > > +		/*
> > > > > +		 * Warning, see cp_leaf_init() comment and rcu_assign_pointer()
> > > > > +		 * documentation.  Since this is a new root, there are no
> > > > > +		 * read-side operations that can view it until it is insert into
> > > > > +		 * the tree after an rcu_assign_pointer() call.
> > > > > +		 */
> > > > > +		RCU_INIT_POINTER(cp->slot[0], mt_mk_node(cp->dst[0].node, mt));
> > > > 
> > > > I just found the above makes my build test using an old version compiler fails.
> > > > Fortunately, seems it is same to the one we discussed before [1], and same
> > > > mitigation like below attached patch works, at least for my test setup.
> > > 
> > > Thanks SJ.
> > 
> > My pleasure :)
> > 
> > > 
> > > This is still with gcc 8.1.0?
> > 
> > Yes.  The test code is available at GitHub [1].
> > 
> > Nonetheless, another test [2] that is using 9.3.0 was also failing.
> 
> You have two failures: one in 8.1 and one in 9.3?

Yes.  FYI, I picked the compiler versions for no good reason but just because
there were reports of DAMON build failures on the compilers in the past.

> 
> I was planning to test 7.5.0 and ensure everything works, but this
> implies my plan will not catch everything?

>From Documentation/process/changes.rst, I find minimal version of GCC for
building kernel is 8.1.  So gcc 7.5.0 might not need to be ensured for?

I have no good expertise on planning tests, though.


Thanks,
SJ

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