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Date: Thu, 30 Jul 2015 11:45:42 -0400
From: Ming Lei <ming.lei@...onical.com>
To: Dave Kleikamp <dave.kleikamp@...cle.com>
Cc: Jens Axboe <axboe@...nel.dk>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Zach Brown <zab@...bo.net>,
Christoph Hellwig <hch@...radead.org>,
Maxim Patlasov <mpatlasov@...allels.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Tejun Heo <tj@...nel.org>, Dave Chinner <david@...morbit.com>,
linux-api@...r.kernel.org
Subject: Re: [PATCH v8 4/6] block: loop: prepare for supporing direct IO
On Thu, Jul 30, 2015 at 11:30 AM, Dave Kleikamp
<dave.kleikamp@...cle.com> 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@...r.kernel.org
>> Signed-off-by: Ming Lei <ming.lei@...onical.com>
>> ---
>> 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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