From: Jan Kara <jack@suse.cz>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Jan Kara <jack@suse.cz>,
"Kirill A. Shutemov" <kirill@shutemov.name>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
kernel-team@fb.com
Subject: Re: [PATCH 5/6] mm: workingset: switch shadow entry tracking to radix tree exceptional counting
Date: Tue, 8 Nov 2016 11:27:16 +0100 [thread overview]
Message-ID: <20161108102716.GL32353@quack2.suse.cz> (raw)
In-Reply-To: <20161107190741.3619-6-hannes@cmpxchg.org>
On Mon 07-11-16 14:07:40, Johannes Weiner wrote:
> Currently, we track the shadow entries in the page cache in the upper
> bits of the radix_tree_node->count, behind the back of the radix tree
> implementation. Because the radix tree code has no awareness of them,
> we rely on random subtleties throughout the implementation (such as
> the node->count != 1 check in the shrinking code which is meant to
> exclude multi-entry nodes, but also happens to skip nodes with only
> one shadow entry since it's accounted in the upper bits). This is
> error prone and has, in fact, caused the bug fixed in d3798ae8c6f3
> ("mm: filemap: don't plant shadow entries without radix tree node").
>
> To remove these subtleties, this patch moves shadow entry tracking
> from the upper bits of node->count to the existing counter for
> exceptional entries. node->count goes back to being a simple counter
> of valid entries in the tree node and can be shrunk to a single byte.
...
> diff --git a/mm/truncate.c b/mm/truncate.c
> index 6ae44571d4c7..d3ce5f261f47 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -53,7 +53,6 @@ static void clear_exceptional_entry(struct address_space *mapping,
> mapping->nrexceptional--;
> if (!node)
> goto unlock;
> - workingset_node_shadows_dec(node);
> /*
> * Don't track node without shadow entries.
> *
> @@ -61,8 +60,7 @@ static void clear_exceptional_entry(struct address_space *mapping,
> * The list_empty() test is safe as node->private_list is
> * protected by mapping->tree_lock.
> */
> - if (!workingset_node_shadows(node) &&
> - !list_empty(&node->private_list))
> + if (!node->exceptional && !list_empty(&node->private_list))
> list_lru_del(&workingset_shadow_nodes,
> &node->private_list);
> __radix_tree_delete_node(&mapping->page_tree, node);
Is this really correct now? The radix tree implementation can move a single
exceptional entry at index 0 from a node into a direct pointer and free
the node while it is still in the LRU list. Or am I missing something?
To fix this I'd prefer to just have a callback from radix tree code when it
is freeing a node, rather that trying to second-guess its implementation in
the page-cache code...
Otherwise the patch looks good to me and I really like the simplification!
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2016-11-08 10:27 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-07 19:07 [PATCH 0/6] mm: workingset: radix tree subtleties & single-page file refaults Johannes Weiner
2016-11-07 19:07 ` [PATCH 1/6] mm: khugepaged: fix radix tree node leak in shmem collapse error path Johannes Weiner
2016-11-08 9:53 ` Jan Kara
2016-11-08 16:12 ` Johannes Weiner
2016-11-09 7:41 ` Jan Kara
2016-11-11 10:59 ` Kirill A. Shutemov
2016-11-11 12:22 ` Jan Kara
2016-11-11 16:37 ` Kirill A. Shutemov
2016-11-14 8:07 ` Jan Kara
2016-11-14 14:29 ` Kirill A. Shutemov
2016-11-14 15:52 ` Johannes Weiner
2016-11-14 16:48 ` Johannes Weiner
2016-11-14 19:40 ` Kirill A. Shutemov
2016-11-15 14:00 ` Johannes Weiner
2016-11-07 19:07 ` [PATCH 2/6] mm: workingset: turn shadow node shrinker bugs into warnings Johannes Weiner
2016-11-08 9:57 ` Jan Kara
2016-11-07 19:07 ` [PATCH 3/6] lib: radix-tree: native accounting of exceptional entries Johannes Weiner
2016-11-08 10:08 ` Jan Kara
2016-11-07 19:07 ` [PATCH 4/6] lib: radix-tree: check accounting of existing slot replacement users Johannes Weiner
2016-11-08 10:12 ` Jan Kara
2016-11-07 19:07 ` [PATCH 5/6] mm: workingset: switch shadow entry tracking to radix tree exceptional counting Johannes Weiner
2016-11-08 10:27 ` Jan Kara [this message]
2016-11-08 19:30 ` Johannes Weiner
2016-11-07 19:07 ` [PATCH 6/6] mm: workingset: restore refault tracking for single-page files Johannes Weiner
2016-11-08 10:31 ` Jan Kara
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