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

[Matthew Wilcox]

On Mon, Mar 30, 2020 at 02:13:50PM +0000, Wei Yang wrote:
> On Mon, Mar 30, 2020 at 06:49:03AM -0700, Matthew Wilcox wrote:
> >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.
> 
> I found this in xarray.rst:
> 
>   Normal pointers may be stored in the XArray directly.  They must be 4-byte
>   aligned, which is true for any pointer returned from kmalloc() and
>   alloc_page().  It isn't true for arbitrary user-space pointers,
>   nor for function pointers.  You can store pointers to statically allocated
>   objects, as long as those objects have an alignment of at least 4.
> 
> So the document here is not correct?

Why do you say that?

(it is slightly out of date; the XArray actually supports storing unaligned
pointers now, but that's not relevant to this discussion)