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SubjectRe: [PATCH 5/5] block: enable dax for raw block devices
On Thu 22-10-15 02:42:11, Dan Williams wrote:
> If an application wants exclusive access to all of the persistent memory
> provided by an NVDIMM namespace it can use this raw-block-dax facility
> to forgo establishing a filesystem. This capability is targeted
> primarily to hypervisors wanting to provision persistent memory for
> guests.
>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Jeff Moyer <jmoyer@redhat.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> fs/block_dev.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 53 insertions(+), 1 deletion(-)
>
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 3255dcec96b4..c27cd1a21a13 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1687,13 +1687,65 @@ static const struct address_space_operations def_blk_aops = {
> .is_dirty_writeback = buffer_check_dirty_writeback,
> };
>
> +#ifdef CONFIG_FS_DAX
> +/*
> + * In the raw block case we do not need to contend with truncation nor
> + * unwritten file extents. Without those concerns there is no need for
> + * additional locking beyond the mmap_sem context that these routines
> + * are already executing under.
> + *
> + * Note, there is no protection if the block device is dynamically
> + * resized (partition grow/shrink) during a fault. A stable block device
> + * size is already not enforced in the blkdev_direct_IO path.
> + *
> + * For DAX, it is the responsibility of the block device driver to
> + * ensure the whole-disk device size is stable while requests are in
> + * flight.
> + *
> + * Finally, these paths do not synchronize against freezing
> + * (sb_start_pagefault(), etc...) since bdev_sops does not support
> + * freezing.

Well, for devices freezing is handled directly in the block layer code
(blk_stop_queue()) since there's no need to put some metadata structures
into a consistent state. So the comment about bdev_sops is somewhat
strange. Otherwise the patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.com>

Honza

> + */
> +static int blkdev_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
> +{
> + return __dax_fault(vma, vmf, blkdev_get_block, NULL);
> +}
> +
> +static int blkdev_dax_pmd_fault(struct vm_area_struct *vma, unsigned long addr,
> + pmd_t *pmd, unsigned int flags)
> +{
> + return __dax_pmd_fault(vma, addr, pmd, flags, blkdev_get_block, NULL);
> +}
> +
> +static const struct vm_operations_struct blkdev_dax_vm_ops = {
> + .page_mkwrite = blkdev_dax_fault,
> + .fault = blkdev_dax_fault,
> + .pmd_fault = blkdev_dax_pmd_fault,
> +};
> +
> +static int blkdev_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> + struct inode *bd_inode = file_bd_inode(file);
> +
> + if (!IS_DAX(bd_inode))
> + return generic_file_mmap(file, vma);
> +
> + file_accessed(file);
> + vma->vm_ops = &blkdev_dax_vm_ops;
> + vma->vm_flags |= VM_MIXEDMAP | VM_HUGEPAGE;
> + return 0;
> +}
> +#else
> +#define blkdev_mmap generic_file_mmap
> +#endif
> +
> const struct file_operations def_blk_fops = {
> .open = blkdev_open,
> .release = blkdev_close,
> .llseek = block_llseek,
> .read_iter = blkdev_read_iter,
> .write_iter = blkdev_write_iter,
> - .mmap = generic_file_mmap,
> + .mmap = blkdev_mmap,
> .fsync = blkdev_fsync,
> .unlocked_ioctl = block_ioctl,
> #ifdef CONFIG_COMPAT
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR


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