linux-kernel - Re: Tagged pointers in the XArray

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Message-ID: <8352b2da-b638-2205-132b-f32893d1cdb7@infradead.org>
Date:   Tue, 28 Aug 2018 16:09:23 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Gao Xiang <gaoxiang25@...wei.com>,
        zhong jiang <zhongjiang@...wei.com>,
        Chao Yu <yuchao0@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: Tagged pointers in the XArray

On 08/28/2018 04:03 PM, Matthew Wilcox wrote:
> On Tue, Aug 28, 2018 at 03:39:01PM -0700, Randy Dunlap wrote:
>> Just a question, please...
>>
>> On 08/28/2018 03:27 PM, Matthew Wilcox wrote:
>>>
>>> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
>>> index c74556ea4258..d1b383f3063f 100644
>>> --- a/include/linux/xarray.h
>>> +++ b/include/linux/xarray.h
>>> @@ -150,6 +150,54 @@ static inline int xa_err(void *entry)
>>>  	return 0;
>>>  }
>>>  
>>> +/**
>>> + * xa_tag_pointer() - Create an XArray entry for a tagged pointer.
>>> + * @p: Plain pointer.
>>> + * @tag: Tag value (0, 1 or 3).
>>> + *
>>
>> What's wrong with a tag value of 2?
> 
> That conflicts with the XArray's internal entries and you get a WARN_ON
> when you try to store it in the array.
> 
>> and what happens when one is used?  [I don't see anything preventing that.]
> 
> Right, there's nothing preventing you from using the value 5 or 19
> or 16777216 either ... I did put in a WARN_ON_ONCE to begin with, but
> decided that was unnecessary.
> 
> Right now our only user uses 0 and 1, so even documenting 3 as a
> possibility isn't _necessary_, but some day somebody is going to want
> to add FILE_NOT_FOUND
> https://thedailywtf.com/articles/What_Is_Truth_0x3f_
> 

Thanks.  :)

-- 
~Randy