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

On Thu, Jul 30, 2015 at 11:30 AM, Dave Kleikamp
[off-list ref] wrote:

On 07/30/2015 06:36 AM, Ming Lei wrote:

quoted

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.

quoted

Cc: linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Signed-off-by: Ming Lei <redacted>
---
 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 */