linux-kernel - Re: [RFC PATCH v4 1/8] iomap: zeroing needs to be pagecache aware

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Date: Fri, 31 May 2024 11:44:57 -0400
From: Brian Foster <bfoster@...hat.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: "Darrick J. Wong" <djwong@...nel.org>,
	Zhang Yi <yi.zhang@...weicloud.com>, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	brauner@...nel.org, david@...morbit.com, chandanbabu@...nel.org,
	jack@...e.cz, willy@...radead.org, yi.zhang@...wei.com,
	chengzhihao1@...wei.com, yukuai3@...wei.com
Subject: Re: [RFC PATCH v4 1/8] iomap: zeroing needs to be pagecache aware

On Fri, May 31, 2024 at 07:05:38AM -0700, Christoph Hellwig wrote:
> On Fri, May 31, 2024 at 07:03:58AM -0700, Darrick J. Wong wrote:
> > > +			/*
> > > +			 * XXX: It would be nice if we could get the offset of
> > > +			 * the next entry in the pagecache so that we don't have
> > > +			 * to iterate one page at a time here.
> > > +			 */
> > > +			offset = offset_in_page(pos);
> > > +			if (bytes > PAGE_SIZE - offset)
> > > +				bytes = PAGE_SIZE - offset;
> > 
> > Why is it PAGE_SIZE here and not folio_size() like below?
> > 
> > (I know you're just copying the existing code; I'm merely wondering if
> > this is some minor bug.)
> 
> See the comment just above :)
> 
> 

FWIW, something like the following is pretty slow with the current
implementation on a quick test:

  xfs_io -fc "falloc -k 0 1t" -c "pwrite 1000g 4k" <file>

.. so I'd think you'd want some kind of data seek or something to more
efficiently process the range.

Brian