Re: [PATCH 1/4] xfs: Don't allocate new buffers on every call to _xfs_buf_find

[Christoph Hellwig <hch@infradead.org>]

> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index c57836d..594cea5 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -171,10 +171,16 @@ STATIC void
>  _xfs_buf_initialize(
>  	xfs_buf_t		*bp,
>  	xfs_buftarg_t		*target,
> -	xfs_off_t		range_base,
> -	size_t			range_length,
> +	xfs_off_t		bno,
> +	size_t			num_blocks,
>  	xfs_buf_flags_t		flags)
>  {
> +	xfs_off_t		range_base;
> +	size_t			range_length;
> +
> +	range_base = BBTOB(bno);
> +	range_length = BBTOB(num_blocks);

What is the point of changing the mostly unrelated _xfs_buf_initialize
prototype in this patch?

I think it (and the other renaming changes related to it) are fine,
but should be a separate patch.  And once you touch _xfs_buf_initialize
after the core of this patch, please merge it with xfs_buf_allocate into
a new xfs_buf_alloc that does the full allocation + initialization and
can also replace xfs_buf_get_empty.

> +	bp = _xfs_buf_find(target, bno, num_blocks, flags, new_bp);
> +	if (!bp) {
> +		xfs_buf_deallocate(new_bp);
> +		return NULL;
> +	}
> +
>  	if (bp == new_bp) {
>  		error = xfs_buf_allocate_memory(bp, flags);
>  		if (error)
>  			goto no_buffer;
> +	} else
>  		xfs_buf_deallocate(new_bp);

I'd recommend moving the call to xfs_buf_allocate_memory into
_xfs_buf_find so that it returns a fully allocated buffer.  In fact I'd
also move the xfs_buf_deallocate(new_bp) into the found side of
_xfs_buf_find, avoiding any conditionals in xfs_buf_get.


>  
> -	XFS_STATS_INC(xb_get);
> -
>  	/*
> -	 * Always fill in the block number now, the mapped cases can do
> -	 * their own overlay of this later.
> +	 * Now we have a workable buffer, fill in the block number so
> +	 * that we can do IO on it.
>  	 */
> -	bp->b_bn = ioff;
> -	bp->b_count_desired = bp->b_buffer_length;
> +	bp->b_bn = bno;

Note that we only need this if we did not find an existing buffer.  It's
not strictly related to the patch, but given that you stop assigning
b_count_desired and redo this whole area it might be worth shifting it
into the if (bp == new_bp) conditional area.


>  
> +found:
> +	ASSERT(bp->b_flags & XBF_MAPPED);

This doesn't look right to me.  Various buffers like inode or remoate attrs
are unmapped, and I can't see any reason why we would assert not beeing
allowed to find them here.

Thinking about it more I'm also not sure skipping the code to map
buffers on a straight cache hit is a good idea - there's nothing
inherent to requiring a given buffer to be mapped for all callers.

_______________________________________________
xfs mailing list
xfs@oss.sgi.com
http://oss.sgi.com/mailman/listinfo/xfs