[PATCH] zram: check bd_openers instead bd_holders

[Minchan Kim]

The bd_holders is increased only when user open the device file
as FMODE_EXCL so if something opens zram0 as !FMODE_EXCL
and request I/O while another user reset zram0, we can see
following warning.

[   30.683449] zram0: detected capacity change from 0 to 64424509440
[   33.736869] Buffer I/O error on dev zram0, logical block 180823, lost async page write
[   33.738814] Buffer I/O error on dev zram0, logical block 180824, lost async page write
[   33.740654] Buffer I/O error on dev zram0, logical block 180825, lost async page write
[   33.742551] Buffer I/O error on dev zram0, logical block 180826, lost async page write
[   33.744153] Buffer I/O error on dev zram0, logical block 180827, lost async page write
[   33.745807] Buffer I/O error on dev zram0, logical block 180828, lost async page write
[   33.747419] Buffer I/O error on dev zram0, logical block 180829, lost async page write
[   33.749060] Buffer I/O error on dev zram0, logical block 180830, lost async page write
[   33.750687] Buffer I/O error on dev zram0, logical block 180831, lost async page write
[   33.752286] Buffer I/O error on dev zram0, logical block 180832, lost async page write
[   33.811590] ------------[ cut here ]------------
[   33.812038] WARNING: CPU: 11 PID: 1996 at fs/block_dev.c:57 __blkdev_put+0x1d7/0x210()
[   33.812817] Modules linked in:
[   33.813142] CPU: 11 PID: 1996 Comm: dd Not tainted 3.19.0-rc6-next-20150202+ #1125
[   33.813837] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[   33.814525]  ffffffff81801a2d ffff880061e77db8 ffffffff815b848e 0000000000000001
[   33.815196]  0000000000000000 ffff880061e77df8 ffffffff8104de2a 0000000000000000
[   33.815867]  ffff88005da287f0 ffff88005da28680 ffff88005da28770 ffff88005da28698
[   33.816536] Call Trace:
[   33.816817]  [<ffffffff815b848e>] dump_stack+0x45/0x57
[   33.817304]  [<ffffffff8104de2a>] warn_slowpath_common+0x8a/0xc0
[   33.817829]  [<ffffffff8104df1a>] warn_slowpath_null+0x1a/0x20
[   33.818331]  [<ffffffff811b60b7>] __blkdev_put+0x1d7/0x210
[   33.818797]  [<ffffffff811b69c0>] blkdev_put+0x50/0x130
[   33.819244]  [<ffffffff811b6b55>] blkdev_close+0x25/0x30
[   33.819723]  [<ffffffff8118079f>] __fput+0xdf/0x1e0
[   33.820140]  [<ffffffff811808ee>] ____fput+0xe/0x10
[   33.820576]  [<ffffffff81068e07>] task_work_run+0xa7/0xe0
[   33.821151]  [<ffffffff81002b89>] do_notify_resume+0x49/0x60
[   33.821721]  [<ffffffff815bf09d>] int_signal+0x12/0x17
[   33.822228] ---[ end trace 274fbbc5664827d2 ]---

The warning comes from bdev_write_node in blkdev_put path.

tatic void bdev_write_inode(struct inode *inode)
{
        spin_lock(&inode->i_lock);
        while (inode->i_state & I_DIRTY) {
                spin_unlock(&inode->i_lock);
                WARN_ON_ONCE(write_inode_now(inode, true)); <========= here.
                spin_lock(&inode->i_lock);
        }
        spin_unlock(&inode->i_lock);
}

The reason is dd process encounters I/O fails due to sudden block device
disappear so in filemap_check_errors in __writeback_single_inode returns
-EIO.

If we checks bd_openners instead of bd_holders, we could address the
problem. When I see the brd, it already have used it rather than
bd_holders so although I'm not a expert of block layer, it seems
to be better.

I can make following warning with below simple script.
In addition, I added msleep(2000) below set_capacity(zram->disk, 0)
after applying your patch to make window huge(Kudos to Ganesh!)

script:

echo $((60<<30)) > /sys/block/zram0/disksize
setsid dd if=/dev/zero of=/dev/zram0 &
sleep 1
setsid echo 1 > /sys/block/zram0/reset

If we checks bd_openners instead of bd_holders, we could address the
problem. When I see the brd, it already have used it rather than
bd_holders so although I'm not a expert of block layer, it seems
to be better.

Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 drivers/block/zram/zram_drv.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index a32069f98afa..cc0e6a3ddb4f 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -811,7 +811,7 @@ static ssize_t reset_store(struct device *dev,
 
 	mutex_lock(&bdev->bd_mutex);
 	/* Do not reset an active device! */
-	if (bdev->bd_holders) {
+	if (bdev->bd_openers) {
 		ret = -EBUSY;
 		goto out;
 	}
-- 
1.9.1



-- 
Kind regards,
Minchan Kim
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