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From: Wei Yang <richard.weiyang@gmail.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Wei Yang <richard.weiyang@gmail.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/9] XArray: internal node is a xa_node when it is bigger than XA_ZERO_ENTRY
Date: Mon, 30 Mar 2020 22:20:13 +0000	[thread overview]
Message-ID: <20200330222013.34nkqen2agujhd6j@master> (raw)
In-Reply-To: <20200330142708.GC22483@bombadil.infradead.org>

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(it is slightly out of date; the XArray actually supports storing unaligned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
Help you, Help me

  reply	other threads:[~2020-03-30 22:20 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-30 12:36 [PATCH 0/9] XArray: several cleanups Wei Yang
2020-03-30 12:36 ` [PATCH 1/9] XArray: fix comment on Zero/Retry entry Wei Yang
2020-03-30 12:46   ` Matthew Wilcox
2020-03-30 13:42     ` Wei Yang
2020-03-30 12:36 ` [PATCH 2/9] XArray: simplify the calculation of shift Wei Yang
2020-03-30 13:20   ` Matthew Wilcox
2020-03-30 14:07     ` Wei Yang
2020-03-30 12:36 ` [PATCH 3/9] XArray: handle a NULL head by itself Wei Yang
2020-03-30 12:36 ` [PATCH 4/9] XArray: don't expect to have more nr_values than count Wei Yang
2020-03-30 12:36 ` [PATCH 5/9] XArray: entry in last level is not expected to be a node Wei Yang
2020-03-30 12:48   ` Matthew Wilcox
2020-03-30 14:15     ` Wei Yang
2020-03-30 14:28       ` Matthew Wilcox
2020-03-30 22:10         ` Wei Yang
2020-03-31 13:42         ` Wei Yang
2020-03-31 16:42           ` Matthew Wilcox
2020-03-31 22:04             ` Wei Yang
2020-03-31 23:59               ` Matthew Wilcox
2020-04-01 22:10                 ` Wei Yang
2020-04-01 22:20                   ` Matthew Wilcox
2020-04-02 12:36                     ` Wei Yang
2020-04-03 22:39                     ` Wei Yang
2020-04-04 15:37                       ` Matthew Wilcox
2020-04-05 11:07                     ` Wei Yang
2020-04-05 21:56                       ` Matthew Wilcox
2020-04-06  1:14                         ` Wei Yang
2020-04-06  1:24     ` Wei Yang
2020-04-11 13:56       ` Wei Yang
2020-04-28 21:24     ` Wei Yang
2020-03-30 12:36 ` [PATCH 6/9] XArray: internal node is a xa_node when it is bigger than XA_ZERO_ENTRY Wei Yang
2020-03-30 12:50   ` Matthew Wilcox
2020-03-30 13:45     ` Wei Yang
2020-03-30 13:49       ` Matthew Wilcox
2020-03-30 14:13         ` Wei Yang
2020-03-30 14:27           ` Matthew Wilcox
2020-03-30 22:20             ` Wei Yang [this message]
2020-03-31  0:06               ` Matthew Wilcox
2020-03-31 13:40                 ` Wei Yang
2020-03-30 12:36 ` [PATCH 7/9] XArray: the NULL xa_node condition is handled in xas_top Wei Yang
2020-03-30 12:36 ` [PATCH 8/9] XArray: take xas_error() handling for clearer logic Wei Yang
2020-03-30 12:36 ` [PATCH 9/9] XArray: adjust xa_offset till it gets the correct node Wei Yang

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