linux-kernel - Re: Write-back from inside FS

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Date:	Sat, 29 Sep 2007 22:10:42 +0300
From:	Artem Bityutskiy <dedekind@...dex.ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Write-back from inside FS - need suggestions

Andrew Morton wrote:
> I'd have thought that a suitable wrapper around a suitably-modified
> sync_sb_inodes() would be appropriate for both filesystems?

Ok, I've modified sync_inodes_sb() so that I can pass it my own wbc,
where I set wcb->nr_to_write = 20. It gives me _exactly_ what I want.
It just flushes a bit more then 20 pages and returns. I use
WB_SYNC_ALL. Great!

Now I would like to understand why it works :-) To my surprise, it
does not deadlock! I call it from ->prepare_write where I'm holding
i_mutex, and it works just fine. It calls ->writepage() without trying
to lock i_mutex! This looks like some witchcraft for me.

This means that if I'm in the middle of an operation or ino #X, I own
its i_mutex, but not I_LOCK, I can be preempted and ->writepage can
be called for a dirty page belonging to this inode #X? I haven't seen
this in practice and I do not believe this may happen. Why?

Could you or someone please give me a hint what exactly
inode->i_flags & I_LOCK protects? What is its relationship to i_mutex?

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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