linux-kernel - Re: [PATCH v10 19/21] xip: Add xip_zero_page

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Message-ID: <20140904210802.GA27730@localhost.localdomain>
Date:	Thu, 4 Sep 2014 17:08:02 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, willy@...ux.intel.com,
	Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH v10 19/21] xip: Add xip_zero_page_range

On Wed, Sep 03, 2014 at 07:21:16PM +1000, Dave Chinner wrote:
> > @@ -481,9 +484,14 @@ int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
> >  		err = dax_get_addr(&bh, &addr, inode->i_blkbits);
> >  		if (err < 0)
> >  			return err;
> > +		/*
> > +		 * ext4 sometimes asks to zero past the end of a block.  It
> > +		 * really just wants to zero to the end of the block.
> > +		 */
> > +		length = min_t(unsigned, length, PAGE_CACHE_SIZE - offset);
> >  		memset(addr + offset, 0, length);
> 
> Sorry, what?
> 
> You introduce that bug with the way dax_truncate_page() is redefined
> to always pass PAGE_CACHE_SIZE a a length later on in this patch.
> into the function. That's hardly an ext4 bug....

ext4 does (or did?) have this bug (expectation?).  I then take advantage
of the fact that we have to accommodate it, so there are now two places
that have to accommodate it.  I forget what the path was that has that
assumption, but xfstests used to display it.

I'm away this week (... bad timing), but I can certainly fix it elsewhere
in ext4 next week.

> >  int dax_clear_blocks(struct inode *, sector_t block, long size);
> > +int dax_zero_page_range(struct inode *, loff_t from, unsigned len, get_block_t);
> >  int dax_truncate_page(struct inode *, loff_t from, get_block_t);
> 
> It's still defined as a function that doesn't exist now....

Oops.

> > +/* Can't be a function because PAGE_CACHE_SIZE is defined in pagemap.h */
> > +#define dax_truncate_page(inode, from, get_block)	\
> > +	dax_zero_page_range(inode, from, PAGE_CACHE_SIZE, get_block)
> 
> And then redefined as a macro here.

Heh, which means we never notice the stale delaration above.  Thanks, C!

> This is wrong, IMO,
> dax_truncate_page() should remain as a function and it should
> correctly calculate how much of the page shoul dbe trimmed, not
> leave landmines that other code has to clean up...
> 
> (Yup, I'm tracking down a truncate bug in XFS from fsx...)

I'll put an assert in the rewrite, make sure that nobody's trying to
overtruncate.

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