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SubjectRe: [PATCH v3 13/15] block, dax: make dax mappings opt-in by default
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On Mon, Nov 2, 2015 at 4:32 PM, Dave Chinner <david@fromorbit.com> wrote:
> On Sun, Nov 01, 2015 at 11:30:53PM -0500, Dan Williams wrote:
>> Now that we have the ability to dynamically enable DAX for a raw block
>> inode, make the behavior opt-in by default. DAX does not have feature
>> parity with pagecache backed mappings, so applications should knowingly
>> enable DAX semantics.
>>
>> Note, this is only for mappings returned to userspace. For the
>> synchronous usages of DAX, dax_do_io(), there is no semantic difference
>> with the bio submission path, so that path remains default enabled.
>>
>> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>> ---
>> block/ioctl.c | 3 +--
>> fs/block_dev.c | 33 +++++++++++++++++++++++----------
>> include/linux/fs.h | 8 ++++++++
>> 3 files changed, 32 insertions(+), 12 deletions(-)
>>
>> diff --git a/block/ioctl.c b/block/ioctl.c
>> index 205d57612fbd..c4c3a09d9ca9 100644
>> --- a/block/ioctl.c
>> +++ b/block/ioctl.c
>> @@ -298,13 +298,12 @@ static inline int is_unrecognized_ioctl(int ret)
>> #ifdef CONFIG_FS_DAX
>> static int blkdev_set_dax(struct block_device *bdev, int n)
>> {
>> - struct gendisk *disk = bdev->bd_disk;
>> int rc = 0;
>>
>> if (n)
>> n = S_DAX;
>>
>> - if (n && !disk->fops->direct_access)
>> + if (n && !blkdev_dax_capable(bdev))
>> return -ENOTTY;
>>
>> mutex_lock(&bdev->bd_inode->i_mutex);
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index 13ce6d0ff7f6..ee34a31e6fa4 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -152,16 +152,37 @@ static struct inode *bdev_file_inode(struct file *file)
>> return file->f_mapping->host;
>> }
>>
>> +#ifdef CONFIG_FS_DAX
>> +bool blkdev_dax_capable(struct block_device *bdev)
>> +{
>> + struct gendisk *disk = bdev->bd_disk;
>> +
>> + if (!disk->fops->direct_access)
>> + return false;
>> +
>> + /*
>> + * If the partition is not aligned on a page boundary, we can't
>> + * do dax I/O to it.
>> + */
>> + if ((bdev->bd_part->start_sect % (PAGE_SIZE / 512))
>> + || (bdev->bd_part->nr_sects % (PAGE_SIZE / 512)))
>> + return false;
>> +
>> + return true;
>
> Where do you check that S_DAX has been enabled on the block device
> now?
>

Only in the mmap path:

static int blkdev_mmap(struct file *file, struct vm_area_struct *vma)
{
struct inode *bd_inode = bdev_file_inode(file);
struct block_device *bdev = I_BDEV(bd_inode);

file_accessed(file);
mutex_lock(&bd_inode->i_mutex);
bdev->bd_map_count++;
if (IS_DAX(bd_inode)) {
vma->vm_ops = &blkdev_dax_vm_ops;
vma->vm_flags |= VM_MIXEDMAP | VM_HUGEPAGE;
} else {
vma->vm_ops = &blkdev_default_vm_ops;
}
mutex_unlock(&bd_inode->i_mutex);

return 0;
}


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