Messages in this thread | | | Subject | Re: [PATCH v3 04/10] fsdax: Introduce dax_iomap_cow_copy() | From | Ritesh Harjani <> | Date | Tue, 23 Mar 2021 21:38:59 +0530 |
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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
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