Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 14 Jun 2010 21:16:29 -0400 Rik van Riel <riel@redhat.com> wrote:

> Would it be hard to add a "please flush this file"
> way to call the filesystem flushing threads?

Passing the igrab()bed inode into the flusher threads would fix the
iput_final() problems, as long as the alloc_pages() caller never blocks
indefinitely waiting for the work which the flusher threads are doing. 

Otherwise we get (very hard-to-hit) deadlocks where the alloc_pages()
caller holds VFS locks and is waiting for the flusher threads while all
the flusher threads are stuck under iput_final() waiting for those VFS
locks.

That's fixable by not using igrab()/iput().  You can use lock_page() to
pin the address_space.  Pass the address of the locked page across to
the flusher threads so they don't try to lock it a second time, or just
use trylocking on that writeback path or whatever.