linux-kernel - Re: [RFC PATCH] iomap: add support to track dirty state of sub pages

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Message-ID: <20200730031934.GA23808@casper.infradead.org>
Date:   Thu, 30 Jul 2020 04:19:34 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     hch@...radead.org, darrick.wong@...cle.com,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [RFC PATCH] iomap: add support to track dirty state of sub pages

On Thu, Jul 30, 2020 at 09:19:01AM +0800, Yu Kuai wrote:
> +++ b/fs/iomap/buffered-io.c
> @@ -29,7 +29,9 @@ struct iomap_page {
>  	atomic_t		read_count;
>  	atomic_t		write_count;
>  	spinlock_t		uptodate_lock;
> +	spinlock_t		dirty_lock;

No need for a separate spinlock.  Just rename uptodate_lock.  Maybe
'bitmap_lock'.

>  	DECLARE_BITMAP(uptodate, PAGE_SIZE / 512);
> +	DECLARE_BITMAP(dirty, PAGE_SIZE / 512);

This is inefficient and poses difficulties for the THP patchset.
Maybe let the discussion on removing the ->uptodate array finish
before posting another patch for review?

> +static void
> +iomap_iop_set_or_clear_range_dirty(
> +	struct page *page,
> +	unsigned int off,
> +	unsigned int len,
> +	bool is_set)

Please follow normal kernel programming style.  This isn't XFS.
Also 'set or clear' with a bool to indicate which to do is horrible
style.  Separate functions!

> @@ -1386,7 +1432,8 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
>  	for (i = 0, file_offset = page_offset(page);
>  	     i < (PAGE_SIZE >> inode->i_blkbits) && file_offset < end_offset;
>  	     i++, file_offset += len) {
> -		if (iop && !test_bit(i, iop->uptodate))
> +		if (iop && (!test_bit(i, iop->uptodate) ||
> +		    !test_bit(i, iop->dirty)))
>  			continue;

Surely we don't need to test ->uptodate here at all.  Why would we write
back a block which isn't dirty?