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
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| @@ -31,6 +31,10 @@ enum { · 30 unmodified lines | |||
| 31 | /* LO_FLAGS that can be cleared using LOOP_SET_STATUS(64) */ | 31 | /* LO_FLAGS that can be cleared using LOOP_SET_STATUS(64) */ |
| 32 | #define LOOP_SET_STATUS_CLEARABLE_FLAGS (LO_FLAGS_AUTOCLEAR) | 32 | #define LOOP_SET_STATUS_CLEARABLE_FLAGS (LO_FLAGS_AUTOCLEAR) |
| 33 | 33 | ||
| 34 | /* LO_FLAGS that can be set using LOOP_CONFIGURE */ | ||
| 35 | #define LOOP_CONFIGURE_SETTABLE_FLAGS (LO_FLAGS_READ_ONLY | LO_FLAGS_AUTOCLEAR \ | ||
| 36 | | LO_FLAGS_PARTSCAN | LO_FLAGS_DIRECT_IO) | ||
| 37 | |||
| 34 | #include <asm/posix_types.h> /* for __kernel_old_dev_t */ | 38 | #include <asm/posix_types.h> /* for __kernel_old_dev_t */ |
| 35 | #include <linux/types.h> /* for __u64 */ | 39 | #include <linux/types.h> /* for __u64 */ |
| 36 | 40 | ||
| @@ -66,6 +70,22 @@ struct loop_info64 { · 29 unmodified lines | |||
| 66 | __u64 lo_init[2]; | 70 | __u64 lo_init[2]; |
| 67 | }; | 71 | }; |
| 68 | 72 | ||
| 73 | /** | ||
| 74 | * struct loop_config - Complete configuration for a loop device. | ||
| 75 | * @fd: fd of the file to be used as a backing file for the loop device. | ||
| 76 | * @block_size: block size to use; ignored if 0. | ||
| 77 | * @info: struct loop_info64 to configure the loop device with. | ||
| 78 | * | ||
| 79 | * This structure is used with the LOOP_CONFIGURE ioctl, and can be used to | ||
| 80 | * atomically setup and configure all loop device parameters at once. | ||
| 81 | */ | ||
| 82 | struct loop_config { | ||
| 83 | __u32 fd; | ||
| 84 | __u32 block_size; | ||
| 85 | struct loop_info64 info; | ||
| 86 | __u64 __reserved[8]; | ||
| 87 | }; | ||
| 88 | |||
| 69 | /* | 89 | /* |
| 70 | * Loop filter types | 90 | * Loop filter types |
| 71 | */ | 91 | */ |
| @@ -96,6 +116,7 @@ struct loop_info64 { · 24 unmodified lines | |||
| 96 | #define LOOP_SET_CAPACITY 0x4C07 | 116 | #define LOOP_SET_CAPACITY 0x4C07 |
| 97 | #define LOOP_SET_DIRECT_IO 0x4C08 | 117 | #define LOOP_SET_DIRECT_IO 0x4C08 |
| 98 | #define LOOP_SET_BLOCK_SIZE 0x4C09 | 118 | #define LOOP_SET_BLOCK_SIZE 0x4C09 |
| 119 | #define LOOP_CONFIGURE 0x4C0A | ||
| 99 | 120 | ||
| 100 | /* /dev/loop-control interface */ | 121 | /* /dev/loop-control interface */ |
| 101 | #define LOOP_CTL_ADD 0x4C80 | 122 | #define LOOP_CTL_ADD 0x4C80 |