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Date: Mon, 1 Feb 2010 17:39:03 +1100
From: Nick Piggin <npiggin@...e.de>
To: Christoph Lameter <cl@...ux-foundation.org>
Cc: Andi Kleen <andi@...stfloor.org>,
Dave Chinner <david@...morbit.com>,
Christoph Lameter <clameter@....com>,
Rik van Riel <riel@...hat.com>,
Pekka Enberg <penberg@...helsinki.fi>,
akpm@...ux-foundation.org, Miklos Szeredi <miklos@...redi.hu>,
Nick Piggin <nickpiggin@...oo.com.au>,
Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org
Subject: Re: buffer heads: Support slab defrag
On Fri, Jan 29, 2010 at 02:49:41PM -0600, Christoph Lameter wrote:
> Defragmentation support for buffer heads. We convert the references to
> buffers to struct page references and try to remove the buffers from
> those pages. If the pages are dirty then trigger writeout so that the
> buffer heads can be removed later.
>
> Reviewed-by: Rik van Riel <riel@...hat.com>
> Signed-off-by: Christoph Lameter <clameter@....com>
> Signed-off-by: Christoph Lameter <cl@...ux-foundation.org>
>
> ---
> fs/buffer.c | 99 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 99 insertions(+)
>
> Index: slab-2.6/fs/buffer.c
> ===================================================================
> --- slab-2.6.orig/fs/buffer.c 2010-01-22 15:09:43.000000000 -0600
> +++ slab-2.6/fs/buffer.c 2010-01-22 16:17:27.000000000 -0600
> @@ -3352,6 +3352,104 @@ int bh_submit_read(struct buffer_head *b
> }
> EXPORT_SYMBOL(bh_submit_read);
>
> +/*
> + * Writeback a page to clean the dirty state
> + */
> +static void trigger_write(struct page *page)
> +{
> + struct address_space *mapping = page_mapping(page);
> + int rc;
> + struct writeback_control wbc = {
> + .sync_mode = WB_SYNC_NONE,
> + .nr_to_write = 1,
> + .range_start = 0,
> + .range_end = LLONG_MAX,
> + .nonblocking = 1,
> + .for_reclaim = 0
> + };
> +
> + if (!mapping->a_ops->writepage)
> + /* No write method for the address space */
> + return;
> +
> + if (!clear_page_dirty_for_io(page))
> + /* Someone else already triggered a write */
> + return;
> +
> + rc = mapping->a_ops->writepage(page, &wbc);
> + if (rc < 0)
> + /* I/O Error writing */
> + return;
> +
> + if (rc == AOP_WRITEPAGE_ACTIVATE)
> + unlock_page(page);
> +}
> +
> +/*
> + * Get references on buffers.
> + *
> + * We obtain references on the page that uses the buffer. v[i] will point to
> + * the corresponding page after get_buffers() is through.
> + *
> + * We are safe from the underlying page being removed simply by doing
> + * a get_page_unless_zero. The buffer head removal may race at will.
> + * try_to_free_buffes will later take appropriate locks to remove the
> + * buffers if they are still there.
> + */
> +static void *get_buffers(struct kmem_cache *s, int nr, void **v)
> +{
> + struct page *page;
> + struct buffer_head *bh;
> + int i, j;
> + int n = 0;
> +
> + for (i = 0; i < nr; i++) {
> + bh = v[i];
> + v[i] = NULL;
> +
> + page = bh->b_page;
> +
> + if (page && PagePrivate(page)) {
> + for (j = 0; j < n; j++)
> + if (page == v[j])
> + continue;
> + }
> +
> + if (get_page_unless_zero(page))
> + v[n++] = page;
This seems wrong to me. The page can have been reused at this
stage.
You technically can't re-check using page->private because that
can be anything and doesn't actually need to be a pointer. You
could re-check bh->b_page, provided that you ensure it is always
cleared before a page is detached, and the correct barriers are
in place.
> + }
> + return NULL;
> +}
> +
> +/*
> + * Despite its name: kick_buffers operates on a list of pointers to
> + * page structs that was set up by get_buffer().
> + */
> +static void kick_buffers(struct kmem_cache *s, int nr, void **v,
> + void *private)
> +{
> + struct page *page;
> + int i;
> +
> + for (i = 0; i < nr; i++) {
> + page = v[i];
> +
> + if (!page || PageWriteback(page))
> + continue;
> +
> + if (trylock_page(page)) {
> + if (PageDirty(page))
> + trigger_write(page);
> + else {
> + if (PagePrivate(page))
> + try_to_free_buffers(page);
> + unlock_page(page);
PagePrivate doesn't necessarily mean it has buffers. try_to_release_page
should be a better idea.
> + }
> + }
> + put_page(page);
> + }
> +}
> +
> static void
> init_buffer_head(void *data)
> {
> @@ -3370,6 +3468,7 @@ void __init buffer_init(void)
> (SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|
> SLAB_MEM_SPREAD),
> init_buffer_head);
> + kmem_cache_setup_defrag(bh_cachep, get_buffers, kick_buffers);
>
> /*
> * Limit the bh occupancy to 10% of ZONE_NORMAL
Buffer heads and buffer head refcounting really stinks badly. Although
I can see the need for a medium term solution until fsblock / some
actual sane refcounting.
--
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