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

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Message-Id: <20200227051753.CD17B42052@d06av24.portsmouth.uk.ibm.com>
Date:   Thu, 27 Feb 2020 10:47:52 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     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 2/26/20 9:47 PM, Darrick J. Wong wrote:
> 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."
> 

:) Got it. Will add more information to it.


> 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.

-extent may return nothing. But the user must still confirm by trying to map
-past this extent, since different filesystems implement this differently.
+extent may return nothing. In some implementations this flag is also set on
+the last dataset queried by the user (via fiemap->fm_length).


Let me know if above looks good.

-ritesh