Re: [RFC PATCH 10/17] block: introduce bdev_dax_direct_access()

[Christoph Hellwig]

On Wed, Feb 01, 2017 at 01:21:40AM -0800, Dan Williams wrote:
> > In/Out parameters are always a bit problematic in terms of API clarity.
> > And updating a device-relative address with an absolute physical one
> > sounds like an odd API for sure.
> 
> Yes, it does, and I thought better of it shortly after sending that. How 
> about:
> 
> long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> unsigned long nr_pages, void **kaddr, pfn_t *pfn)

Yes, that looks good to me.
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Re: [RFC PATCH 10/17] block: introduce bdev_dax_direct_access()

[Dan Williams]

On Wed, Feb 1, 2017 at 12:10 AM, Christoph Hellwig  wrote:
> On Mon, Jan 30, 2017 at 10:16:29AM -0800, Dan Williams wrote:
>> Ok, now that dax_map_atomic() is gone, it's much easier to remove
>> struct blk_dax_ctl.
>>
>> We can also move the partition alignment checks to be a one-time check
>> at bdev_dax_capable() time and kill bdev_dax_direct_access() in favor
>> of calling dax_direct_access() directly.
>
> Yes, please.
>
>> >> + if ((sector + DIV_ROUND_UP(dax->size, 512))
>> >> + > part_nr_sects_read(bdev->bd_part))
>> >> + return -ERANGE;
>> >> + sector += get_start_sect(bdev);
>> >> + return dax_direct_access(dax_inode, sector * 512, >addr,
>> >> + >pfn, dax->size);
>> >
>> > And please switch to using bytes as the granularity given that we're
>> > deadling with byte addressable memory.
>>
>> dax_direct_access() does take a byte aligned physical address, but it
>> needs to be at least page aligned since we are returning a pfn_t...
>>
>> Hmm, perhaps the input should be raw page frame number. We could
>> reduce one of the arguments by making the current 'pfn_t *' parameter
>> an in/out-parameter.
>
> In/Out parameters are always a bit problematic in terms of API clarity.
> And updating a device-relative address with an absolute physical one
> sounds like an odd API for sure.

Yes, it does, and I thought better of it shortly after sending that. How about:

long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
unsigned long nr_pages, void **kaddr, pfn_t *pfn)
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Re: [RFC PATCH 10/17] block: introduce bdev_dax_direct_access()

[Christoph Hellwig]

On Mon, Jan 30, 2017 at 10:16:29AM -0800, Dan Williams wrote:
> Ok, now that dax_map_atomic() is gone, it's much easier to remove
> struct blk_dax_ctl.
> 
> We can also move the partition alignment checks to be a one-time check
> at bdev_dax_capable() time and kill bdev_dax_direct_access() in favor
> of calling dax_direct_access() directly.

Yes, please.

> >> + if ((sector + DIV_ROUND_UP(dax->size, 512))
> >> + > part_nr_sects_read(bdev->bd_part))
> >> + return -ERANGE;
> >> + sector += get_start_sect(bdev);
> >> + return dax_direct_access(dax_inode, sector * 512, >addr,
> >> + >pfn, dax->size);
> >
> > And please switch to using bytes as the granularity given that we're
> > deadling with byte addressable memory.
> 
> dax_direct_access() does take a byte aligned physical address, but it
> needs to be at least page aligned since we are returning a pfn_t...
> 
> Hmm, perhaps the input should be raw page frame number. We could
> reduce one of the arguments by making the current 'pfn_t *' parameter
> an in/out-parameter.

In/Out parameters are always a bit problematic in terms of API clarity.
And updating a device-relative address with an absolute physical one
sounds like an odd API for sure.
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Re: [RFC PATCH 10/17] block: introduce bdev_dax_direct_access()

[Dan Williams]

On Mon, Jan 30, 2017 at 4:32 AM, Christoph Hellwig  wrote:
> On Sat, Jan 28, 2017 at 12:36:58AM -0800, Dan Williams wrote:
>> Provide a replacement for bdev_direct_access() that uses
>> dax_operations.direct_access() instead of
>> block_device_operations.direct_access(). Once all consumers of the old
>> api have been converted bdev_direct_access() will be deleted.
>>
>> Given that block device partitioning decisions can cause dax page
>> alignment constraints to be violated we still need to validate the
>> block_device before calling the dax ->direct_access method.
>>
>> Signed-off-by: Dan Williams 
>> ---
>>  block/Kconfig  |1 +
>>  drivers/dax/super.c|   33 +
>>  fs/block_dev.c |   28 
>>  include/linux/blkdev.h |3 +++
>>  include/linux/dax.h|2 ++
>>  5 files changed, 67 insertions(+)
>>
>> diff --git a/block/Kconfig b/block/Kconfig
>> index 8bf114a3858a..9be785173280 100644
>> --- a/block/Kconfig
>> +++ b/block/Kconfig
>> @@ -6,6 +6,7 @@ menuconfig BLOCK
>> default y
>> select SBITMAP
>> select SRCU
>> +   select DAX
>> help
>>Provide block layer support for the kernel.
>>
>> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
>> index eb844ffea3cf..ab5b082df5dd 100644
>> --- a/drivers/dax/super.c
>> +++ b/drivers/dax/super.c
>> @@ -65,6 +65,39 @@ struct dax_inode {
>>   const struct dax_operations *ops;
>>  };
>>
>> +long dax_direct_access(struct dax_inode *dax_inode, phys_addr_t dev_addr,
>> + void **kaddr, pfn_t *pfn, long size)
>> +{
>> + long avail;
>> +
>> + /*
>> +  * The device driver is allowed to sleep, in order to make the
>> +  * memory directly accessible.
>> +  */
>> + might_sleep();
>> +
>> + if (!dax_inode)
>> + return -EOPNOTSUPP;
>> +
>> + if (!dax_inode_alive(dax_inode))
>> + return -ENXIO;
>> +
>> + if (size < 0)
>> + return size;
>> +
>> + if (dev_addr % PAGE_SIZE)
>> + return -EINVAL;
>> +
>> + avail = dax_inode->ops->direct_access(dax_inode, dev_addr, kaddr, pfn,
>> + size);
>> + if (!avail)
>> + return -ERANGE;
>> + if (avail > 0 && avail & ~PAGE_MASK)
>> + return -ENXIO;
>> + return min(avail, size);
>> +}
>> +EXPORT_SYMBOL_GPL(dax_direct_access);
>> +
>>  bool dax_inode_alive(struct dax_inode *dax_inode)
>>  {
>>   lockdep_assert_held(_srcu);
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index edb1d2b16b8f..bf4b51a3a412 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -18,6 +18,7 @@
>>  #include 
>>  #include 
>>  #include 
>> +#include 
>>  #include 
>>  #include 
>>  #include 
>> @@ -763,6 +764,33 @@ long bdev_direct_access(struct block_device *bdev, 
>> struct blk_dax_ctl *dax)
>>  EXPORT_SYMBOL_GPL(bdev_direct_access);
>>
>>  /**
>> + * bdev_dax_direct_access() - bdev-sector to pfn_t and kernel virtual 
>> address
>> + * @bdev: host block device for @dax_inode
>> + * @dax_inode: interface data and operations for a memory device
>> + * @dax: control and output parameters for ->direct_access
>> + *
>> + * Return: negative errno if an error occurs, otherwise the number of bytes
>> + * accessible at this address.
>> + *
>> + * Locking: must be called with dax_read_lock() held
>> + */
>> +long bdev_dax_direct_access(struct block_device *bdev,
>> + struct dax_inode *dax_inode, struct blk_dax_ctl *dax)
>> +{
>> + sector_t sector = dax->sector;
>> +
>> + if (!blk_queue_dax(bdev->bd_queue))
>> + return -EOPNOTSUPP;
>
> I don't think this should take a bdev - the caller should know if
> it has a dax_inode.  Also if you touch this anyway can we kill
> the annoying struct blk_dax_ctl calling convention?  Passing the
> four arguments explicitly is just a lot more readable and understandable.

Ok, now that dax_map_atomic() is gone, it's much easier to remove
struct blk_dax_ctl.

We can also move the partition alignment checks to be a one-time check
at bdev_dax_capable() time and kill bdev_dax_direct_access() in favor
of calling dax_direct_access() directly.

>> + if ((sector + DIV_ROUND_UP(dax->size, 512))
>> + > part_nr_sects_read(bdev->bd_part))
>> + return -ERANGE;
>> + sector += get_start_sect(bdev);
>> + return dax_direct_access(dax_inode, sector * 512, >addr,
>> + >pfn, dax->size);
>
> And please switch to using bytes as the granularity given that we're
> deadling with byte addressable memory.

dax_direct_access() does take a byte aligned physical address, but it
needs to be at least page aligned since we are returning a pfn_t...

Hmm, perhaps the input should be raw page frame number. We could
reduce one of the arguments by making the current 'pfn_t *' parameter
an in/out-parameter.
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