linux-kernel - Re: [PATCH] maple_tree: update mas_next[

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Message-ID: <6fgs2mcvwlitkjza5d7cpu3mk34sqqn53vqazkicyge4gtqt7f@5dvqmpubypip>
Date: Mon, 26 Jan 2026 15:20:17 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Ballance <andrewjballance@...il.com>,
        maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] maple_tree: update mas_next[_range] docs

* Alice Ryhl <aliceryhl@...gle.com> [260121 04:56]:
> On Tue, Jan 20, 2026 at 12:54:47PM -0500, Liam R. Howlett wrote:
> > * Alice Ryhl <aliceryhl@...gle.com> [260118 06:00]:
> > > If you read the docs, it sounds like the difference between these
> > > functions is whether mas->index and mas->last are updated. However, if
> > > you read the implementation, you will instead find that the difference
> > > is whether NULL entries are skipped.
> > 
> > This is not the intent.
> > 
> > mas_ should return special values including the XA_ZERO_ENTRY.
> > 
> > mas_next() should get the next non-NULL value.
> > 
> > mas_next_range() should advance the maple state to the next range,
> > regardless of what is in the range (NULL, special, or a regular entry).
> > 
> > Both should update the mas->index and mas->last values, if it moves
> > (ie, no error state is encountered).
> 
> I guess I'm a bit confused about the difference between XA_ZERO_ENTRY
> and returning NULL. Isn't the case where we return NULL when a slot has
> been reserved but not inserted yet?

mas_ will return the special entries.

mtree_ will return NULL on special entries.  I think this is just
mtree_load().

If you want to use your own locking and use mas_, then you can filter
out the special entries yourself.

If you want to use the normal api, then the special entries are filtered
for you.

This way you can mix/match the apis but the noral api still remains
simple to use - even if there are advanced users that mixed in.

The idea is that if you're using the advanced interface and storing
special entries, then you probably want to do something different on
those entries - at least sometimes.

> 
> Like the docs, you use "get" vs "advance" wording here, but I don't
> think there's any difference behavior-wise? Is one intended?

On return type, no, there isn't a difference.  The difference is where
the mas points in the end (mas->offset, mas->index, mas->last).

If a NULL is encountered bu mas_next(), then we proceed to the next slot
(which must have a value, if there is a next slot).  So, mas_next() will
always return the next entry until there is not a next entry - then it
returns NULL.  Note that mas_next() takes an 'end' value where we'll
stop advancing slots regardless if there are values.

If a NULL is encountered by mas_next_range(), then we return the NULL.
So, in this way, we can move to the next range even if it's NULL.

I hope this makes the difference more clear?

Thanks,
Liam