linux-ext4 - Re: [PATCHv3 6/6] Documentation: Correct the description of FIEMAP_EXTENT

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Date:   Wed, 26 Feb 2020 08:17:42 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Ritesh Harjani <riteshh@...ux.ibm.com>, jack@...e.cz,
        tytso@....edu, linux-ext4@...r.kernel.org,
        adilger.kernel@...ger.ca, linux-fsdevel@...r.kernel.org,
        hch@...radead.org, cmaiolino@...hat.com
Subject: Re: [PATCHv3 6/6] Documentation: Correct the description of
 FIEMAP_EXTENT_LAST

On Wed, Feb 26, 2020 at 05:05:03AM -0800, Matthew Wilcox wrote:
> On Wed, Feb 26, 2020 at 03:27:08PM +0530, Ritesh Harjani wrote:
> > Currently FIEMAP_EXTENT_LAST is not working consistently across
> > different filesystem's fiemap implementations and thus this feature
> > may be broken. So fix the documentation about this flag to meet the
> > right expectations.
> 
> Are you saying filesystems have both false positives and false negatives?
> I can understand how a filesystem might fail to set FIEMAP_EXTENT_LAST,
> but not how a filesystem might set it when there's actually another
> extent beyond this one.
> 
> >  * FIEMAP_EXTENT_LAST
> > -This is the last extent in the file. A mapping attempt past this
> > -extent will return nothing.
> > +This is generally the last extent in the file. A mapping attempt past this
> > +extent may return nothing. But the user must still confirm by trying to map
> > +past this extent, since different filesystems implement this differently.

"This flag means nothing and can be set arbitrarily by the fs for the lulz."

Yuck.  I was really hoping for "This is set on the last extent record in
the dataset generated by the query parameters", particularly becaue
that's how e2fsprogs utilties interpret that flag.

--D