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Date:   Fri, 24 Nov 2017 17:50:41 +0100
From:   Martin Steigerwald <>
To:     Matthew Wilcox <>
        Matthew Wilcox <>
Subject: Re: XArray documentation

Hello Matthew.

Matthew Wilcox - 24.11.17, 02:16:
> ======
> XArray
> ======
> Overview
> ========
> The XArray is an array of ULONG_MAX entries.  Each entry can be either
> a pointer, or an encoded value between 0 and LONG_MAX.  It is efficient
> when the indices used are densely clustered; hashing the object and
> using the hash as the index will not perform well.  A freshly-initialised
> XArray contains a NULL pointer at every index.  There is no difference
> between an entry which has never been stored to and an entry which has most
> recently had NULL stored to it.

I am no kernel developer (just provided a tiny bit of documentation a long 
time ago)… but on reading into this, I missed:

What is it about? And what is it used for?

"Overview" appears to be already a description of the actual implementation 
specifics, instead of… well an overview.

Of course, I am sure you all know what it is for… but someone who wants to 
learn about the kernel is likely to be confused by such a start.


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