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

From: Andrew Morton
Date: Tue Jun 15 2010 - 00:39:04 EST

On Tue, 15 Jun 2010 00:08:14 -0400 Rik van Riel <riel@xxxxxxxxxx> wrote:

> On 06/14/2010 09:45 PM, Andrew Morton wrote:
> > On Mon, 14 Jun 2010 21:16:29 -0400 Rik van Riel<riel@xxxxxxxxxx> 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.
> Any thread that does not have __GFP_FS set in its gfp_mask
> cannot wait for the flusher to complete. This is regardless
> of the mechanism used to kick the flusher.

mm... kinda. A bare order-zero __GFP_WAIT allocation can still wait
forever, afaict.

> Then again, those threads cannot call ->writepage today
> either, so we should be fine keeping that behaviour.

I'm not sure. iput_final() can take a lot of locks, both VFS and
heaven knows what within the individual filesystems. Is it the case
that all allocations which occur under all of those locks is always
!__GFP_FS? Hard to say...

> Threads that do have __GFP_FS in their gfp_mask can wait
> for the flusher in various ways. Maybe the lock_page()
> method can be simplified by having the flusher thread
> unlock the page the moment it gets it, and then run the
> normal flusher code?

Well, _something_ has to pin the address_space. A single locked page
will do.

> The pageout code (in shrink_page_list) already unlocks
> the page anyway before putting it back on the relevant
> LRU list. It would be easy enough to skip that unlock
> and let the flusher thread take care of it.

Once that page is unlocked, we can't touch *mapping - its inode can be
concurrently reclaimed. Although I guess the technique in
handle_write_error() can be reused.

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