Re: [PATCH] Fix queueing work if !bdi_cap_writeback_dirty()

[OGAWA Hirofumi]

Fengguang Wu <fengguang.wu@xxxxxxxxx> writes:

> On Wed, Sep 12, 2012 at 03:28:42AM +0900, OGAWA Hirofumi wrote:
>> 
>> If bdi has BDI_CAP_NO_WRITEBACK, bdi_forker_thread() doesn't start
>> writeback thread. This means there is no consumer of work item made
>> by bdi_queue_work().
>> 
>> This adds to checking of !bdi_cap_writeback_dirty(sb->s_bdi) before
>> calling bdi_queue_work(), otherwise queued work never be consumed.
>> 
>> Signed-off-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
>> ---
>> 
>>  fs/fs-writeback.c |    7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>> 
>> diff -puN fs/fs-writeback.c~noop_backing_dev_info-check-fix fs/fs-writeback.c
>> --- linux/fs/fs-writeback.c~noop_backing_dev_info-check-fix	2012-09-11 06:12:30.000000000 +0900
>> +++ linux-hirofumi/fs/fs-writeback.c	2012-09-11 06:12:30.000000000 +0900
>> @@ -120,6 +120,9 @@ __bdi_start_writeback(struct backing_dev
>>  {
>>  	struct wb_writeback_work *work;
>>  
>> +	if (!bdi_cap_writeback_dirty(bdi))
>> +		return;
>
> Will someone in the current kernel actually call
> __bdi_start_writeback() on a BDI_CAP_NO_WRITEBACK bdi?
>
> If the answer is no, VM_BUG_ON(!bdi_cap_writeback_dirty(bdi)) looks better.

I guess nobody call it in current kernel though. Hmm.., but we also have
check in __mark_inode_dirty(), nobody should be using it, right?

If we defined it as the bug, I can't see what BDI_CAP_NO_WRITEBACK wants
to do actually.  We are not going to allow to disable the writeback task?

I was going to use this to disable writeback task on my developing FS...

Thanks.
-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
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