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Date:   Fri, 2 Oct 2020 08:15:05 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     "Darrick J. Wong" <darrick.wong@oracle.com>
Cc:     linux-xfs@vger.kernel.org
Subject: Re: [PATCH 2/2] xfs: streamline xfs_getfsmap performance
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On Thu, Oct 01, 2020 at 09:49:30PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Refactor xfs_getfsmap to improve its performance: instead of indirectly
> calling a function that copies one record to userspace at a time, create
> a shadow buffer in the kernel and copy the whole array once at the end.
> This should speed it up significantly.

So we save an indirect call and uaccess_enable/disable per entry,
but pay for it with the cost of a memory allocation (and actually
using that memory).

Doesn't seem like an obvious win to me, do you have numbers?