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: <20200331134020.xejcx3mggobmzmji@master>
Date:   Tue, 31 Mar 2020 13:40:21 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Wei Yang <richard.weiyang@...il.com>,
        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 05:06:49PM -0700, Matthew Wilcox wrote:
>On Mon, Mar 30, 2020 at 10:20:13PM +0000, Wei Yang wrote:
>> 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?
>
>But also incorrect, because it'll be different on different architectures.
>It's 4096.  That's all.
>
>>   * a node is an internal entry, do we need to compare with xa_mk_internal()
>>     instead?
>
>No.  4096 is better because it's a number which is easily expressible in
>many CPU instruction sets.  4094 is much less likely to be an easy number
>to encode.
>
>> >(it is slightly out of date; the XArray actually supports storing unaligned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.
>
>Did you want to send a patch?

I am not clear how it supports unaligned pointers. So maybe not now.

Actually, I still not get the point between page size and valid pointer. Why a
valid pointer couldn't be less than 4096? The first page in address space is
handled differently? Maybe I miss some point. I'd appreciate it if you'd share
some light.

Thanks
-- 
Wei Yang
Help you, Help me