2023-09-21 20:16:41

by Matthew Wilcox

[permalink] [raw]
Subject: Re: [PATCH 2/2] maple_tree: Add MAS_UNDERFLOW and MAS_OVERFLOW states

On Thu, Sep 21, 2023 at 02:12:36PM -0400, Liam R. Howlett wrote:
> Since MAS_NONE is used for handling of the maple tree when it's a single
> entry at 0 (just a pointer), changing the handling of MAS_NONE in
> mas_find() would make the code more complicated and error prone.

Single entry at index 0 is MAS_ROOT, not MAS_NONE.


2023-09-21 22:52:16

by Liam R. Howlett

[permalink] [raw]
Subject: Re: [PATCH 2/2] maple_tree: Add MAS_UNDERFLOW and MAS_OVERFLOW states

* Matthew Wilcox <[email protected]> [230921 14:40]:
> On Thu, Sep 21, 2023 at 02:12:36PM -0400, Liam R. Howlett wrote:
> > Since MAS_NONE is used for handling of the maple tree when it's a single
> > entry at 0 (just a pointer), changing the handling of MAS_NONE in
> > mas_find() would make the code more complicated and error prone.
>
> Single entry at index 0 is MAS_ROOT, not MAS_NONE.

Ah, sorry. I didn't explain this well. We end up in MAS_NONE when we
search from MAS_ROOT upwards.. that is, there's a value only at 0 and we
request 1 - ULONG_MAX, or we've called mas_find() with an index > 0. So
there is no node in the tree for this entry.

The complication arises when mas_prev(), mas_next() or
mas_walk()/mas_find() has already set MAS_NONE, then we can't tell the
difference and so we don't really know what the Right Thing to do would
be.