On 13.05.2020 15.38, Martijn Coenen wrote:
This allows userspace to completely setup a loop device with a single
ioctl, removing the in-between state where the device can be partially
configured - eg the loop device has a backing file associated with it,
but is reading from the wrong offset.
Besides removing the intermediate state, another big benefit of this
ioctl is that LOOP_SET_STATUS can be slow; the main reason for this
slowness is that LOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to
freeze the associated queue; this requires waiting for RCU
synchronization, which I've measured can take about 15-20ms on this
device on average.
In addition to doing what LOOP_SET_STATUS can do, LOOP_CONFIGURE can
also be used to:
- Set the correct block size immediately by setting
loop_config.block_size (avoids LOOP_SET_BLOCK_SIZE)
- Explicitly request direct I/O mode by setting LO_FLAGS_DIRECT_IO
in loop_config.info.lo_flags (avoids LOOP_SET_DIRECT_IO)
- Explicitly request read-only mode by setting LO_FLAGS_READ_ONLY
in loop_config.info.lo_flags
Here's setting up ~70 regular loop devices with an offset on an x86
Android device, using LOOP_SET_FD and LOOP_SET_STATUS:
vsoc_x86:/system/apex # time for i in `seq 30 100`;
do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
0m03.40s real 0m00.02s user 0m00.03s system
Here's configuring ~70 devices in the same way, but using a modified
losetup that uses the new LOOP_CONFIGURE ioctl:
vsoc_x86:/system/apex # time for i in `seq 30 100`;
do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
0m01.94s real 0m00.01s user 0m00.01s system
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Martijn Coenen <maco@xxxxxxxxxxx>
---
drivers/block/loop.c | 104 ++++++++++++++++++++++++++++----------
include/uapi/linux/loop.h | 21 ++++++++
2 files changed, 97 insertions(+), 28 deletions(-)
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 13518ba191f5..a565c5aafa52 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -228,6 +228,19 @@ static void __loop_update_dio(struct loop_device *lo, bool dio)
blk_mq_unfreeze_queue(lo->lo_queue);
}
+/**
+ * loop_validate_block_size() - validates the passed in block size
+ * @bsize: size to validate
+ */
+static int
+loop_validate_block_size(unsigned short bsize)
+{
+ if (bsize < 512 || bsize > PAGE_SIZE || !is_power_of_2(bsize))
+ return -EINVAL;
+
+ return 0;
+}
+
/**
* loop_set_size() - sets device size and notifies userspace
* @lo: struct loop_device to set the size for
@@ -1050,23 +1063,24 @@ loop_set_status_from_info(struct loop_device *lo,
return 0;
}
-static int loop_set_fd(struct loop_device *lo, fmode_t mode,
- struct block_device *bdev, unsigned int arg)
+static int loop_configure(struct loop_device *lo, fmode_t mode,
+ struct block_device *bdev,
+ const struct loop_config *config)
{
struct file *file;
struct inode *inode;
struct address_space *mapping;
struct block_device *claimed_bdev = NULL;
- int lo_flags = 0;
int error;
loff_t size;
bool partscan;
+ unsigned short bsize;
/* This is safe, since we have a reference from open(). */
__module_get(THIS_MODULE);
error = -EBADF;
- file = fget(arg);
+ file = fget(config->fd);
if (!file)
goto out;
@@ -1075,7 +1089,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
* here to avoid changing device under exclusive owner.
*/
if (!(mode & FMODE_EXCL)) {
- claimed_bdev = bd_start_claiming(bdev, loop_set_fd);
+ claimed_bdev = bd_start_claiming(bdev, loop_configure);
if (IS_ERR(claimed_bdev)) {
error = PTR_ERR(claimed_bdev);
goto out_putf;
@@ -1097,11 +1111,26 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
mapping = file->f_mapping;
inode = mapping->host;
+ size = get_loop_size(lo, file);
+
+ if ((config->info.lo_flags & ~LOOP_CONFIGURE_SETTABLE_FLAGS) != 0) {
+ error = -EINVAL;
+ goto out_unlock;
+ }
+
+ if (config->block_size) {
+ error = loop_validate_block_size(config->block_size);
+ if (error)
+ goto out_unlock;
+ }
+
+ error = loop_set_status_from_info(lo, &config->info);
loop_set_status_from_info() doesn't call loop_config_discard() nor does
loop_configure(), I assume it should be called?
I'm asking because I upgraded to util-linux 2.37 which use the new
LOOP_CONFIGURE ioctl and fstrim is complaining that "the discard
operation is not supported" and the missing call of
loop_config_discard() stood out. Sadly I haven't been able to reproduce
the issue reliably outside CI (yet).
+ if (error)
+ goto out_unlock;
+
if (!(file->f_mode & FMODE_WRITE) || !(mode & FMODE_WRITE) ||
!file->f_op->write_iter)
- lo_flags |= LO_FLAGS_READ_ONLY;
-
- size = get_loop_size(lo, file);
+ lo->lo_flags |= LO_FLAGS_READ_ONLY;
error = loop_prepare_queue(lo);
if (error)
