linux-kernel - Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded

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Date:	Thu, 12 Mar 2009 12:25:24 +0000
From:	David Howells <dhowells@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	dhowells@...hat.com, torvalds@...ux-foundation.org,
	peterz@...radead.org, Enrik.Berkhan@...com,
	uclinux-dev@...inux.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded

Andrew Morton <akpm@...ux-foundation.org> wrote:

> Was there a specific reason for using the low-level SetPageDirty()?
> 
> On the write() path, ramfs pages will be dirtied by
> simple_commit_write()'s set_page_dirty(), which calls
> __set_page_dirty_no_writeback().
>
> It just so happens that __set_page_dirty_no_writeback() is equivalent
> to a simple SetPageDirty() - it bypasses all the extra things which we
> do for normal permanent-storage-backed pages.
> 
> But I'd have thought that it would be cleaner and more maintainable (albeit
> a bit slower) to go through the a_ops?

It basically boils down to SetPageDirty() with extra overhead, which you
pointed out.  We're manually manipulating the pagecache for this inode anyway,
so does it matter?

The main thing I think I'd rather get rid of is:

		if (!pagevec_add(&lru_pvec, page))
			__pagevec_lru_add_file(&lru_pvec);
	...
	pagevec_lru_add_file(&lru_pvec);

Which as Peter points out:

	The ramfs stuff is rather icky in that it adds the pages to the aging
	list, marks them dirty, but does not provide a writeout method. 

	This will make the paging code scan over them (continuously) trying to
	clean them, failing that (lack of writeout method) and putting them back
	on the list.

Not requiring the pages to be added to the LRU would be a really good idea.
They are not discardable, be it in MMU or NOMMU mode, except when the inode
itself is discarded.

Furthermore, does it really make sense for ramfs to use do_sync_read/write()
and generic_file_aio_read/write(), at least for NOMMU-mode?  These add a lot
of overhead, and ramfs doesn't really do either direct I/O or AIO.

The main point in favour of using these routines is commonality; but they do
add a lot of layers of overhead.  Does ramfs read/write performance matter
than much, I wonder.

David
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