Re: [PATCH 6/9] XArray: internal node is a xa_node when it is bigger than XA_ZERO_ENTRY

Matthew Wilcox
2020-03-30T13:49:05Z

On Mon, Mar 30, 2020 at 01:45:19PM +0000, Wei Yang wrote:
> On Mon, Mar 30, 2020 at 05:50:06AM -0700, Matthew Wilcox wrote:
> >On Mon, Mar 30, 2020 at 12:36:40PM +0000, Wei Yang wrote:
> >> As the comment mentioned, we reserved several ranges of internal node
> >> for tree maintenance, 0-62, 256, 257. This means a node bigger than
> >> XA_ZERO_ENTRY is a normal node.
> >>
> >> The checked on XA_ZERO_ENTRY seems to be more meaningful.
> >
> >257-1023 are also reserved, they just aren't used yet. XA_ZERO_ENTRY
> >is not guaranteed to be the largest reserved entry.
>
> Then why we choose 4096?

Because 4096 is the smallest page size supported by Linux, so we're
guaranteed that anything less than 4096 is not a valid pointer.