Re: [PATCH v10 19/21] xip: Add xip_zero_page_range

Matthew Wilcox
2014-09-04T21:08:11Z

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.