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

[OGAWA Hirofumi]

Fengguang Wu <fengguang.wu@xxxxxxxxx> writes:

>> The writeback task is always called with sync_mode != WB_SYNC_ALL except
>> sync_inodes_sb(). But FS has sb->s_op->sync_fs() handler for
>> sync_inodes_sb() path. So, writeback task just bothers FS to control to
>> flush.
>> 
>> Also it wants to control the reclaimable of inode cache too, because FS
>> have to control to flush, and wants to use inode in own FS task, and it
>> knows when inode is cleaned and can be reclaimed.
>> 
>> I thought there are 2 options - 1) pin inode with iget(), and iput() on
>> own FS task, 2) disable writeback task and care about inode reclaim by
>> dirty flags.
>> 
>> (1) was complex (e.g. inode can be the orphan inode), and seems to be
>> ineffective workaround to survive with writeback task.
>
> In principle, the VFS should of course give enough flexibility for the
> FS. But it's all about the details that matter. As for the
> BDI_CAP_NO_WRITEBACK approach, I'm afraid you'll not get the expected
> "FS control" through it. Because the flusher thread may already have a
> long queue of works which will take long time to finish. It even have
> its internal background/periodic works that's not controllable this
> way, see wb_check_background_flush().

wb_check_background_flush() is called from,

bdi_forker_thread()
    bdi_writeback_thread()
        wb_do_writeback()
            wb_check_background_flush()

But, bdi_forker_thread() never start bdi_writeback_thread() if
!bdi_cap_writeback_dirty(bdi).

Or I'm seeing something wrong here?

> And BDI_CAP_NO_WRITEBACK is expected to be a static/constant flag that
> always evaluate to true/false for a given bdi.  There will be
> correctness problems if you change the BDI_CAP_NO_WRITEBACK flag
> dynamically.

I'm going to use it as static or per-sb by initialized in
fill_super(). And it uses always BDI_CAP_NO_WRITEBACK if sb is
available. Because own FS task flush instead.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
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