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Subject[PATCH] loop: avoid EAGAIN, if offset or block_size are changed
Date
Previously, there was a bug where user could see stale buffer cache (e.g, 512B)
attached in the 4KB-sized pager cache, when the block size was changed from
512B to 4KB. That was fixed by:
commit 5db470e229e2 ("loop: drop caches if offset or block_size are changed")

But, there were some regression reports saying the fix returns EAGAIN easily.
So, this patch removes previously added EAGAIN condition, nrpages != 0.

Instead, it changes the flow like this:
- sync_blockdev()
- blk_mq_freeze_queue()
: change the loop configuration
- blk_mq_unfreeze_queue()
- sync_blockdev()
- invalidate_bdev()

After invalidating the buffer cache, we must see the full valid 4KB page.

Additional concern came from Bart in which we can lose some data when
changing the lo_offset. In that case, this patch adds:
- sync_blockdev()
- blk_set_queue_dying
- blk_mq_freeze_queue()
: change the loop configuration
- blk_mq_unfreeze_queue()
- blk_queue_flag_clear(QUEUE_FLAG_DYING);
- sync_blockdev()
- invalidate_bdev()

Report: https://bugs.chromium.org/p/chromium/issues/detail?id=938958#c38

Cc: <stable@vger.kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: Bart Van Assche <bvanassche@acm.org>
Fixes: 5db470e229e2 ("loop: drop caches if offset or block_size are changed")
Reported-by: Gwendal Grignou <gwendal@chromium.org>
Reported-by: grygorii tertychnyi <gtertych@cisco.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
drivers/block/loop.c | 65 ++++++++++++++++++++++----------------------
1 file changed, 33 insertions(+), 32 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 739b372a5112..8c9da7f9b1f6 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1245,6 +1245,8 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
kuid_t uid = current_uid();
struct block_device *bdev;
bool partscan = false;
+ bool drop_request = false;
+ bool drop_cache = false;

err = mutex_lock_killable(&loop_ctl_mutex);
if (err)
@@ -1264,14 +1266,21 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
goto out_unlock;
}

+ if (lo->lo_offset != info->lo_offset)
+ drop_request = true;
if (lo->lo_offset != info->lo_offset ||
- lo->lo_sizelimit != info->lo_sizelimit) {
- sync_blockdev(lo->lo_device);
- kill_bdev(lo->lo_device);
- }
+ lo->lo_sizelimit != info->lo_sizelimit)
+ drop_cache = true;

- /* I/O need to be drained during transfer transition */
- blk_mq_freeze_queue(lo->lo_queue);
+ sync_blockdev(lo->lo_device);
+
+ if (drop_request) {
+ blk_set_queue_dying(lo->lo_queue);
+ blk_mq_freeze_queue_wait(lo->lo_queue);
+ } else {
+ /* I/O need to be drained during transfer transition */
+ blk_mq_freeze_queue(lo->lo_queue);
+ }

err = loop_release_xfer(lo);
if (err)
@@ -1298,14 +1307,6 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)

if (lo->lo_offset != info->lo_offset ||
lo->lo_sizelimit != info->lo_sizelimit) {
- /* kill_bdev should have truncated all the pages */
- if (lo->lo_device->bd_inode->i_mapping->nrpages) {
- err = -EAGAIN;
- pr_warn("%s: loop%d (%s) has still dirty pages (nrpages=%lu)\n",
- __func__, lo->lo_number, lo->lo_file_name,
- lo->lo_device->bd_inode->i_mapping->nrpages);
- goto out_unfreeze;
- }
if (figure_loop_size(lo, info->lo_offset, info->lo_sizelimit)) {
err = -EFBIG;
goto out_unfreeze;
@@ -1342,6 +1343,8 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)

out_unfreeze:
blk_mq_unfreeze_queue(lo->lo_queue);
+ if (drop_request)
+ blk_queue_flag_clear(QUEUE_FLAG_DYING, lo->lo_queue);

if (!err && (info->lo_flags & LO_FLAGS_PARTSCAN) &&
!(lo->lo_flags & LO_FLAGS_PARTSCAN)) {
@@ -1350,6 +1353,12 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
bdev = lo->lo_device;
partscan = true;
}
+
+ /* truncate stale pages cached by previous operations */
+ if (!err && drop_cache) {
+ sync_blockdev(lo->lo_device);
+ invalidate_bdev(lo->lo_device);
+ }
out_unlock:
mutex_unlock(&loop_ctl_mutex);
if (partscan)
@@ -1531,7 +1540,7 @@ static int loop_set_dio(struct loop_device *lo, unsigned long arg)

static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
{
- int err = 0;
+ bool drop_cache = false;

if (lo->lo_state != Lo_bound)
return -ENXIO;
@@ -1539,31 +1548,23 @@ static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
if (arg < 512 || arg > PAGE_SIZE || !is_power_of_2(arg))
return -EINVAL;

- if (lo->lo_queue->limits.logical_block_size != arg) {
- sync_blockdev(lo->lo_device);
- kill_bdev(lo->lo_device);
- }
+ if (lo->lo_queue->limits.logical_block_size != arg)
+ drop_cache = true;

+ sync_blockdev(lo->lo_device);
blk_mq_freeze_queue(lo->lo_queue);
-
- /* kill_bdev should have truncated all the pages */
- if (lo->lo_queue->limits.logical_block_size != arg &&
- lo->lo_device->bd_inode->i_mapping->nrpages) {
- err = -EAGAIN;
- pr_warn("%s: loop%d (%s) has still dirty pages (nrpages=%lu)\n",
- __func__, lo->lo_number, lo->lo_file_name,
- lo->lo_device->bd_inode->i_mapping->nrpages);
- goto out_unfreeze;
- }
-
blk_queue_logical_block_size(lo->lo_queue, arg);
blk_queue_physical_block_size(lo->lo_queue, arg);
blk_queue_io_min(lo->lo_queue, arg);
loop_update_dio(lo);
-out_unfreeze:
blk_mq_unfreeze_queue(lo->lo_queue);

- return err;
+ /* truncate stale pages cached by previous operations */
+ if (drop_cache) {
+ sync_blockdev(lo->lo_device);
+ invalidate_bdev(lo->lo_device);
+ }
+ return 0;
}

static int lo_simple_ioctl(struct loop_device *lo, unsigned int cmd,
--
2.25.1.481.gfbce0eb801-goog
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