linux-kernel - Re: [PATCH v5 22/22] XIP: Add support for unwritten extents

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Date:	Thu, 23 Jan 2014 12:13:48 -0700 (MST)
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Matthew Wilcox <willy@...ux.intel.com>
cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v5 22/22] XIP: Add support for unwritten extents

On Thu, 23 Jan 2014, Matthew Wilcox wrote:
> On Wed, Jan 22, 2014 at 03:51:56PM -0700, Ross Zwisler wrote:
> > > +			if (hole) {
> > >  				addr = NULL;
> > > -				hole = true;
> > >  				size = bh->b_size;
> > > +			} else {
> > > +				unsigned first;
> > > +				retval = xip_get_addr(inode, bh, &addr);
> > > +				if (retval < 0)
> > > +					break;
> > > +				size = retval;
> > > +				first = offset - (block << inode->i_blkbits);
> > > +				if (buffer_unwritten(bh))
> > > +					memset(addr, 0, first);
> > > +				addr += first;
> > 
> > +                               size -= first;
> > 
> > This is needed so that we don't overrun the XIP buffer we are given in the
> > event that our user buffer >= our XIP buffer and the start of our I/O isn't
> > block aligned.
> 
> You're right!  Thank you!  However, we also need it for the hole ==
> true case, don't we?  So maybe something like this, incrementally on top of
> patch 22/22:
> 
> P.S. Can someone come up with a better name for this variable than 'first'?
> I'd usually use 'offset', but that's already taken.  'annoying_bit' seems a
> bit judgemental.  'misaligned', maybe?  'skip' or 'seek' like dd uses?
> 
> diff --git a/fs/xip.c b/fs/xip.c
> index 92157ff..1ae00db 100644
> --- a/fs/xip.c
> +++ b/fs/xip.c
> @@ -103,6 +103,7 @@ static ssize_t xip_io(int rw, struct inode *inode, const struct iovec *iov,
>  
>  		if (max == offset) {
>  			sector_t block = offset >> inode->i_blkbits;
> +			unsigned first = offset - (block << inode->i_blkbits);
>  			long size;
>  			memset(bh, 0, sizeof(*bh));
>  			bh->b_size = ALIGN(end - offset, PAGE_SIZE);
> @@ -121,14 +122,12 @@ static ssize_t xip_io(int rw, struct inode *inode, const struct iovec *iov,
>  
>  			if (hole) {
>  				addr = NULL;
> -				size = bh->b_size;
> +				size = bh->b_size - first;
>  			} else {
> -				unsigned first;
>  				retval = xip_get_addr(inode, bh, &addr);
>  				if (retval < 0)
>  					break;
> -				size = retval;
> -				first = offset - (block << inode->i_blkbits);
> +				size = retval - first;
>  				if (buffer_unwritten(bh))
>  					memset(addr, 0, first);
>  				addr += first;

Yep, this seems right to me.

Maybe "misalignment"?  Seems more descriptive (if a bit long), but I don't
know if there are other, better existing conventions.

- Ross
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