On Fri, Feb 22, 2019 at 10:21 AM Darrick J. Wong
<darrick.wong@xxxxxxxxxx> wrote:
>
> Hi all!
>
> Uh, we have an internal customer <cough> who's been trying out MAP_SYNC
> on pmem, and they've observed that one has to do a fair amount of
> legwork (in the form of mkfs.xfs parameters) to get the kernel to set up
> 2M PMD mappings. They (of course) want to mmap hundreds of GB of pmem,
> so the PMD mappings are much more efficient.
>
> I started poking around w.r.t. what mkfs.xfs was doing and realized that
> if the fsdax pmem device advertised iomin/ioopt of 2MB, then mkfs will
> set up all the parameters automatically. Below is my ham-handed attempt
> to teach the kernel to do this.
>
> Comments, flames, "WTF is this guy smoking?" are all welcome. :)
>
> --D
>
> ---
> Configure pmem devices to advertise the default page alignment when said
> block device supports fsdax. Certain filesystems use these iomin/ioopt
> hints to try to create aligned file extents, which makes it much easier
> for mmaps to take advantage of huge page table entries.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
> drivers/nvdimm/pmem.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index bc2f700feef8..3eeb9dd117d5 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -441,8 +441,11 @@ static int pmem_attach_disk(struct device *dev,
> blk_queue_logical_block_size(q, pmem_sector_size(ndns));
> blk_queue_max_hw_sectors(q, UINT_MAX);
> blk_queue_flag_set(QUEUE_FLAG_NONROT, q);
> - if (pmem->pfn_flags & PFN_MAP)
> + if (pmem->pfn_flags & PFN_MAP) {
> blk_queue_flag_set(QUEUE_FLAG_DAX, q);
> + blk_queue_io_min(q, PFN_DEFAULT_ALIGNMENT);
> + blk_queue_io_opt(q, PFN_DEFAULT_ALIGNMENT);
The device alignment might sometimes be bigger than this default.
Would there be any detrimental effects for filesystems if io_min and
io_opt were set to 1GB?
I'm thinking and xfs-realtime configuration might be able to support
1GB mappings in the future.
