Re: [PATCH v8 4/6] block: loop: prepare for supporing direct IO

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On Thu, Jul 30, 2015 at 11:30 AM, Dave Kleikamp
<dave.kleikamp@xxxxxxxxxx> wrote:
> On 07/30/2015 06:36 AM, Ming Lei wrote:
>> This patches provides one interface for enabling direct IO
>> from user space:
>>
>>       - userspace(such as losetup) can pass 'file' which is
>>       opened/fcntl as O_DIRECT
>>
>> Also __loop_update_dio() is introduced to check if direct I/O
>> can be used on current loop setting.
>>
>> The last big change is to introduce LO_FLAGS_DIRECT_IO flag
>> for userspace to know if direct IO is used to access backing
>> file.
>
> lo->use_dio and LO_FLAGS_DIRECT_IO seem redundant. Wouldn't it be
> simpler to use one or the other?

losetup is a stateless utility, which means it need to retrieve the dio status
via the flag of LO_FLAGS_DIRECT_IO, which is one API change.

>
>>
>> Cc: linux-api@xxxxxxxxxxxxxxx
>> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
>> ---
>>  drivers/block/loop.c      | 63 ++++++++++++++++++++++++++++++++++++++++++++++-
>>  drivers/block/loop.h      |  2 ++
>>  include/uapi/linux/loop.h |  1 +
>>  3 files changed, 65 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
>> index 1875aad..799cc23 100644
>> --- a/drivers/block/loop.c
>> +++ b/drivers/block/loop.c
>> @@ -164,6 +164,47 @@ static loff_t get_loop_size(struct loop_device *lo, struct file *file)
>>       return get_size(lo->lo_offset, lo->lo_sizelimit, file);
>>  }
>>
>> +static void __loop_update_dio(struct loop_device *lo, bool dio)
>> +{
>> +     struct file *file = lo->lo_backing_file;
>> +     struct inode *inode = file->f_mapping->host;
>> +     bool use_dio;
>> +     unsigned dio_align = inode->i_sb->s_bdev ?
>> +             (bdev_io_min(inode->i_sb->s_bdev) - 1) : 0;
>> +
>> +     /*
>> +      * We support direct I/O only if lo_offset is aligned
>> +      * with the min I/O size of backing device.
>> +      *
>> +      * Request's offset and size will be checked in I/O path.
>> +      */
>> +     if (dio) {
>> +             if (!dio_align || (lo->lo_offset & dio_align))
>> +                     use_dio = false;
>> +             else
>> +                     use_dio = true;
>> +     } else {
>> +             use_dio = false;
>> +     }
>> +
>> +     /* flush dirty pages before changing direct IO */
>> +     vfs_fsync(file, 0);
>> +
>> +     /*
>> +      * The flag of LO_FLAGS_DIRECT_IO is handled similarly with
>> +      * LO_FLAGS_READ_ONLY, both are set from kernel, and losetup
>> +      * will get updated by ioctl(LOOP_GET_STATUS)
>> +      */
>> +     blk_mq_freeze_queue(lo->lo_queue);
>> +     lo->use_dio = use_dio;
>> +     lo->dio_align = dio_align;
>> +     if (use_dio)
>> +             lo->lo_flags |= LO_FLAGS_DIRECT_IO;
>> +     else
>> +             lo->lo_flags &= ~LO_FLAGS_DIRECT_IO;
>> +     blk_mq_unfreeze_queue(lo->lo_queue);
>> +}
>> +
>>  static int
>>  figure_loop_size(struct loop_device *lo, loff_t offset, loff_t sizelimit)
>>  {
>> @@ -173,8 +214,12 @@ figure_loop_size(struct loop_device *lo, loff_t offset, loff_t sizelimit)
>>
>>       if (unlikely((loff_t)x != size))
>>               return -EFBIG;
>> -     if (lo->lo_offset != offset)
>> +     if (lo->lo_offset != offset) {
>>               lo->lo_offset = offset;
>> +
>> +             /* update dio if lo_offset is changed*/
>> +             __loop_update_dio(lo, lo->use_dio);
>> +     }
>>       if (lo->lo_sizelimit != sizelimit)
>>               lo->lo_sizelimit = sizelimit;
>>       set_capacity(lo->lo_disk, x);
>> @@ -421,6 +466,11 @@ struct switch_request {
>>       struct completion wait;
>>  };
>>
>> +static inline void loop_update_dio(struct loop_device *lo)
>> +{
>> +     __loop_update_dio(lo, io_is_direct(lo->lo_backing_file));
>> +}
>> +
>>  /*
>>   * Do the actual switch; called from the BIO completion routine
>>   */
>> @@ -441,6 +491,7 @@ static void do_loop_switch(struct loop_device *lo, struct switch_request *p)
>>               mapping->host->i_bdev->bd_block_size : PAGE_SIZE;
>>       lo->old_gfp_mask = mapping_gfp_mask(mapping);
>>       mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
>> +     loop_update_dio(lo);
>>  }
>>
>>  /*
>> @@ -627,11 +678,19 @@ static ssize_t loop_attr_partscan_show(struct loop_device *lo, char *buf)
>>       return sprintf(buf, "%s\n", partscan ? "1" : "0");
>>  }
>>
>> +static ssize_t loop_attr_dio_show(struct loop_device *lo, char *buf)
>> +{
>> +     int dio = (lo->lo_flags & LO_FLAGS_DIRECT_IO);
>> +
>> +     return sprintf(buf, "%s\n", dio ? "1" : "0");
>> +}
>> +
>>  LOOP_ATTR_RO(backing_file);
>>  LOOP_ATTR_RO(offset);
>>  LOOP_ATTR_RO(sizelimit);
>>  LOOP_ATTR_RO(autoclear);
>>  LOOP_ATTR_RO(partscan);
>> +LOOP_ATTR_RO(dio);
>>
>>  static struct attribute *loop_attrs[] = {
>>       &loop_attr_backing_file.attr,
>> @@ -639,6 +698,7 @@ static struct attribute *loop_attrs[] = {
>>       &loop_attr_sizelimit.attr,
>>       &loop_attr_autoclear.attr,
>>       &loop_attr_partscan.attr,
>> +     &loop_attr_dio.attr,
>>       NULL,
>>  };
>>
>> @@ -783,6 +843,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
>>       if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
>>               blk_queue_flush(lo->lo_queue, REQ_FLUSH);
>>
>> +     loop_update_dio(lo);
>>       set_capacity(lo->lo_disk, size);
>>       bd_set_size(bdev, size << 9);
>>       loop_sysfs_init(lo);
>> diff --git a/drivers/block/loop.h b/drivers/block/loop.h
>> index b6c7d21..63f8e14 100644
>> --- a/drivers/block/loop.h
>> +++ b/drivers/block/loop.h
>> @@ -58,6 +58,8 @@ struct loop_device {
>>       struct mutex            lo_ctl_mutex;
>>       struct kthread_worker   worker;
>>       struct task_struct      *worker_task;
>> +     unsigned                dio_align;
>> +     bool                    use_dio;
>>
>>       struct request_queue    *lo_queue;
>>       struct blk_mq_tag_set   tag_set;
>> diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
>> index e0cecd2..949851c 100644
>> --- a/include/uapi/linux/loop.h
>> +++ b/include/uapi/linux/loop.h
>> @@ -21,6 +21,7 @@ enum {
>>       LO_FLAGS_READ_ONLY      = 1,
>>       LO_FLAGS_AUTOCLEAR      = 4,
>>       LO_FLAGS_PARTSCAN       = 8,
>> +     LO_FLAGS_DIRECT_IO      = 16,
>>  };
>>
>>  #include <asm/posix_types.h> /* for __kernel_old_dev_t */
>>
--
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