linux-kernel - Re: [PATCH 6/9] XArray: internal node is a xa_node when it is bigger than XA_ZERO

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Message-ID: <20200330134903.GB22483@bombadil.infradead.org>
Date:   Mon, 30 Mar 2020 06:49:03 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/9] XArray: internal node is a xa_node when it is bigger
 than XA_ZERO_ENTRY

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.