linux-kernel - Re: Tagged pointers in the XArray

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Message-ID: <20180828230329.GE11400@bombadil.infradead.org>
Date:   Tue, 28 Aug 2018 16:03:29 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Randy Dunlap <rdunlap@...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 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_