linux-ext4 - Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super

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Date:	Fri, 19 Nov 2010 09:54:24 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	Mark Lord <kernel@...savvy.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Greg Freemyer <greg.freemyer@...il.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	James Bottomley <James.Bottomley@...e.de>,
	Jeff Moyer <jmoyer@...hat.com>,
	Matthew Wilcox <matthew@....cx>,
	Josef Bacik <josef@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>, tytso@....edu,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, sandeen@...hat.com
Subject: Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate
 super_operation

On Fri, Nov 19, 2010 at 09:48:12AM -0500, Mark Lord wrote:
> I'm not sure about the issues on "adapting the block layer" ?
> For FITRIM, the blocks being trimmed would be reserved at the fs level,
> before issuing the discard for them.  So ordering through the block layer
> shouldn't matter much for it.  Does that simplify things?
> 
> I see FITRIM just allocating a page to hold the ranges (for the >1 case)
> and passing that page down through the layers to libata (or any other
> LLD that supports >1 ranges).

Ordering should not be an issue.  What were problems when I tried this
before is that we currently assume in the block layer that discard
bios have a valid bi_sector/bi_size, which is already needed e.g. for
the trivial remapping use for partitions and that they don't have
a payload.  You'd need to teach various places that discard payloads
may have a payload, which contains multiple ranges that have a
sector/len tuple that needs to be remapped and checked in various
places.

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