From: Christoph Hellwig <hch@infradead.org>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Christoph Hellwig <hch@infradead.org>,
xfs <linux-xfs@vger.kernel.org>,
linux-api@vger.kernel.org,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-ext4 <linux-ext4@vger.kernel.org>
Subject: Re: [PATCH] fsmap: fix documentation of FMR_OF_LAST
Date: Sun, 3 Sep 2017 01:41:02 -0700 [thread overview]
Message-ID: <20170903084102.GC32385@infradead.org> (raw)
In-Reply-To: <20170901150716.GW3775@magnolia>
On Fri, Sep 01, 2017 at 08:07:16AM -0700, Darrick J. Wong wrote:
> fmh_entries is the number of records returned, not the number of records
> in the dataset. If, for example, you allocate space for 100 records and
> perform a query for a block that has been reflinked 1000 times, the
> dataset size is 1000 but fmh_entries is set to 100. The lack of a LAST
> flag on the 100th record tells you that there's more records to return.
>
> If however you allocate space for 100 records and the block is reflinked
> exactly 100 times, there's no way (without the flag) for userspace to
> know that record 100 is the end of the dataset, so the only thing it can
> do is to fsmap_advance() and try the query again, only to receive zero
> results. Granted I don't think fsmap queries are all /that/ expensive,
> but it's trivial for the kernel to set the flag.
Oh, ok - it had to look up this defintion of dataset first. With
that the explanation makes total sense:
Reviewed-by: Christoph Hellwig <hch@lst.de>
prev parent reply other threads:[~2017-09-03 8:41 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-31 20:49 [PATCH] fsmap: fix documentation of FMR_OF_LAST Darrick J. Wong
2017-09-01 0:30 ` Dave Chinner
2017-09-01 8:09 ` Christoph Hellwig
[not found] ` <20170901080904.GA32746-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
2017-09-01 15:07 ` Darrick J. Wong
2017-09-03 8:41 ` Christoph Hellwig [this message]
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