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

[Wei Yang]

On Mon, Mar 30, 2020 at 07:27:08AM -0700, Matthew Wilcox wrote:
>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.
>> 

So you want to say, the 4096 makes sure XArray will not store an address in
first page? If this is the case, I have two suggestions:

  * use PAGE_SIZE would be more verbose?
  * a node is an internal entry, do we need to compare with xa_mk_internal()
    instead?

And also suggest to add a comment on this, otherwise it seems a little magic.

>> 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(it is slightly out of date; the XArray actually supports storing unaligned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)

Ok, maybe this document need to update.

-- 
Wei Yang
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