On Mon, Nov 30, 2015 at 8:20 AM, Raghavendra K T
<raghavendra.kt@xxxxxxxxxxxxxxxxxx> wrote:
> * Ilya Dryomov <idryomov@xxxxxxxxx> [2015-11-20 22:22:34]:
>
>> Since 52ebea749aae ("writeback: make backing_dev_info host
>> cgroup-specific bdi_writebacks") inode, at some point in its lifetime,
>> gets attached to a wb (struct bdi_writeback). Detaching happens on
>> evict, in inode_detach_wb() called from __destroy_inode(), and involves
>> updating wb.
>>
>> However, detaching an internal bdev inode from its wb in
>> __destroy_inode() is too late. Its bdi and by extension root wb are
>> embedded into struct request_queue, which has different lifetime rules
>> and can be freed long before the final bdput() is called (can be from
>> __fput() of a corresponding /dev inode, through dput() - evict() -
>> bd_forget(). bdevs hold onto the underlying disk/queue pair only while
>> opened; as soon as bdev is closed all bets are off. In fact,
>> disk/queue can be gone before __blkdev_put() even returns:
>>
>> 1499 static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
>> 1500 {
>> ...
>> 1518 if (bdev->bd_contains == bdev) {
>> 1519 if (disk->fops->release)
>> 1520 disk->fops->release(disk, mode);
>>
>> [ Driver puts its references to disk/queue ]
>>
>> 1521 }
>> 1522 if (!bdev->bd_openers) {
>> 1523 struct module *owner = disk->fops->owner;
>> 1524
>> 1525 disk_put_part(bdev->bd_part);
>> 1526 bdev->bd_part = NULL;
>> 1527 bdev->bd_disk = NULL;
>> 1528 if (bdev != bdev->bd_contains)
>> 1529 victim = bdev->bd_contains;
>> 1530 bdev->bd_contains = NULL;
>> 1531
>> 1532 put_disk(disk);
>>
>> [ We put ours, the queue is gone
>> The last bdput() would result in a write to invalid memory ]
>>
>> 1533 module_put(owner);
>> ...
>> 1539 }
>>
>> Since bdev inodes are special anyway, detach them in __blkdev_put()
>> after clearing inode's dirty bits, turning the problematic
>> inode_detach_wb() in __destroy_inode() into a noop.
>>
>> add_disk() grabs its disk->queue since 523e1d399ce0 ("block: make
>> gendisk hold a reference to its queue"), so the old ->release comment
>> is removed in favor of the new inode_detach_wb() comment.
>>
>> Cc: stable@xxxxxxxxxxxxxxx # 4.2+, needs backporting
>> Signed-off-by: Ilya Dryomov <idryomov@xxxxxxxxx>
>> ---
> Feel free to add
> Tested-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
>
> I was facing bad memory access problem while creating thousands of containers.
> With this patch I am able to create more than 10k containers without hitting
> the problem.
> I had reported the problem here: https://lkml.org/lkml/2015/11/19/149
Great! Christoph's concern is with ->i_wb as a whole, not this
particular patch - Al, this one is marked for stable, can we get it
merged into -rc4? Or should it go through Jens' tree, as cgroup
writeback patches did?
Thanks,
Ilya
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