linux-kernel - Re: [PATCH 3/6] writeback: make nr_to

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Date:	Wed, 4 May 2011 05:42:21 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Dave Chinner <david@...morbit.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 3/6] writeback: make nr_to_write a per-file limit

On Wed, May 04, 2011 at 05:17:10PM +0800, Wu Fengguang wrote:
> This ensures large dirty files can be written in the full 4MB writeback
> chunk size, rather than whatever remained quota in wbc->nr_to_write.

I like the high-level idea, but the implementation of overriding
nr_to_write and then copying it back seems rather ugly.

The basic problem seems to be that struct writeback_control is
designed to control writeback of a single file, but we keep abuse it
for writing multiple files in writeback_sb_inodes and its callers.

It seems like we should only build the struct writeback_control from
struct wb_writeback_work down in writeback_sb_inodes, even if that
means passing some more information to it either in struct
wb_writeback_work or on the stack.

Then writeback_sb_inodes can do something like

	if (wbc.sync_mode == WB_SYNC_NONE)
		wbc.nr_to_write = min(MAX_WRITEBACK_PAGES, work->nr_pages);
	else
		wbc.nr_to_write = LONG_MAX;

for each inode it writes.

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