Re: [PATCH v3 04/10] fsdax: Introduce dax_iomap_cow_copy()

[Ritesh Harjani <riteshh@linux.ibm.com>]

On 3/19/21 7:22 AM, Shiyang Ruan wrote:
> In the case where the iomap is a write operation and iomap is not equal
> to srcmap after iomap_begin, we consider it is a CoW operation.
> 
> The destance extent which iomap indicated is new allocated extent.
> So, it is needed to copy the data from srcmap to new allocated extent.
> In theory, it is better to copy the head and tail ranges which is
> outside of the non-aligned area instead of copying the whole aligned
> range. But in dax page fault, it will always be an aligned range.  So,
> we have to copy the whole range in this case.
> 
> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>   fs/dax.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
>   1 file changed, 66 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index a70e6aa285bb..181aad97136a 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1037,6 +1037,51 @@ static int dax_iomap_direct_access(struct iomap *iomap, loff_t pos, size_t size,
>   	return rc;
>   }
>   
> +/*
> + * Copy the head and tail part of the pages not included in the write but
> + * required for CoW, because pos/pos+length are not page aligned.  But in dax
> + * page fault case, the range is page aligned, we need to copy the whole range
> + * of data.  Use copy_edge to distinguish these cases.
> + */


Is this version better? Feel free to update/change.

dax_iomap_cow_copy(): This can be called from two places.
Either during DAX write fault, to copy the length size data to daddr.
Or, while doing normal DAX write operation, dax_iomap_actor() might call 
this to do the copy of either start or end unaligned address. In this 
case the rest of the copy of aligned ranges is taken care by 
dax_iomap_actor() itself.
Also, note DAX fault will always result in aligned pos and pos + length.

* @pos:		address to do copy from.
* @length:	size of copy operation.
* @align_size:	aligned w.r.t align_size (either PMD_SIZE or PAGE_SIZE)
* @srcmap:	iomap srcmap
* @daddr: 	destination address to copy to.

> +static int dax_iomap_cow_copy(loff_t pos, loff_t length, size_t align_size,
> +		struct iomap *srcmap, void *daddr, bool copy_edge)

do we need bool copy_edge here?
We can detect non-align case directly if head_off != pos or pd_end != 
end no?


> +{
> +	loff_t head_off = pos & (align_size - 1);
> +	size_t size = ALIGN(head_off + length, align_size);
> +	loff_t end = pos + length;
> +	loff_t pg_end = round_up(end, align_size);
> +	void *saddr = 0;
> +	int ret = 0;
> +
> +	ret = dax_iomap_direct_access(srcmap, pos, size, &saddr, NULL);
> +	if (ret)
> +		return ret;
> +
> +	if (!copy_edge)
> +		return copy_mc_to_kernel(daddr, saddr, length);
> +
> +	/* Copy the head part of the range.  Note: we pass offset as length. */
> +	if (head_off) {
> +		if (saddr)
> +			ret = copy_mc_to_kernel(daddr, saddr, head_off);
> +		else
> +			memset(daddr, 0, head_off);
> +	}
> +	/* Copy the tail part of the range */
> +	if (end < pg_end) {
> +		loff_t tail_off = head_off + length;
> +		loff_t tail_len = pg_end - end;
> +
> +		if (saddr)
> +			ret = copy_mc_to_kernel(daddr + tail_off,
> +					saddr + tail_off, tail_len);
> +		else
> +			memset(daddr + tail_off, 0, tail_len);
> +	}

Can you pls help me understand in which case where saddr is 0 and we
will fall back to memset API ?
I was thinking shouldn't such restrictions be coded inside 
copy_mc_to_kernel() function in general?


-ritesh