From: Andrew Morton <akpm@linux-foundation.org>
To: Hugh Dickins <hughd@google.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 1/12] radix_tree: exceptional entries and indices
Date: Sat, 18 Jun 2011 14:48:32 -0700 [thread overview]
Message-ID: <20110618144832.cfc665b0.akpm@linux-foundation.org> (raw)
In-Reply-To: <alpine.LSU.2.00.1106171702150.1345@sister.anvils>
On Fri, 17 Jun 2011 17:13:38 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:
> On Fri, 17 Jun 2011, Andrew Morton wrote:
> > On Tue, 14 Jun 2011 03:42:27 -0700 (PDT)
> > Hugh Dickins <hughd@google.com> wrote:
> >
> > > The low bit of a radix_tree entry is already used to denote an indirect
> > > pointer, for internal use, and the unlikely radix_tree_deref_retry() case.
> > > Define the next bit as denoting an exceptional entry, and supply inline
> > > functions radix_tree_exception() to return non-0 in either unlikely case,
> > > and radix_tree_exceptional_entry() to return non-0 in the second case.
> >
> > Yes, the RADIX_TREE_INDIRECT_PTR hack is internal-use-only, and doesn't
> > operate on (and hence doesn't corrupt) client-provided items.
> >
> > This patch uses bit 1 and uses it against client items, so for
> > practical purpoese it can only be used when the client is storing
> > addresses. And it needs new APIs to access that flag.
> >
> > All a bit ugly. Why not just add another tag for this? Or reuse an
> > existing tag if the current tags aren't all used for these types of
> > pages?
>
> I couldn't see how to use tags without losing the "lockless" lookups:
So lockless pagecache broke the radix-tree tag-versus-item coherency as
well as the address_space nrpages-vs-radix-tree coherency. Isn't it
fun learning these things.
> because the tag is a separate bit from the entry itself, unless you're
> under tree_lock, there would be races when changing from page pointer
> to swap entry or back, when slot was updated but tag not or vice versa.
So... take tree_lock? What effect does that have? It'd better be
"really bad", because this patchset does nothing at all to improve core
MM maintainability :(
next prev parent reply other threads:[~2011-06-18 21:48 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-14 10:40 [PATCH 0/12] tmpfs: convert from old swap vector to radix tree Hugh Dickins
2011-06-14 10:42 ` [PATCH 1/12] radix_tree: exceptional entries and indices Hugh Dickins
2011-06-14 11:22 ` Pekka Enberg
2011-06-15 0:24 ` Hugh Dickins
2011-06-17 23:38 ` Andrew Morton
2011-06-18 0:07 ` Randy Dunlap
2011-06-18 0:12 ` Randy Dunlap
2011-06-18 1:52 ` Hugh Dickins
2011-07-19 22:36 ` Hugh Dickins
2011-07-19 23:28 ` Randy Dunlap
2011-06-18 0:13 ` Hugh Dickins
2011-06-18 21:48 ` Andrew Morton [this message]
2011-07-12 22:56 ` Hugh Dickins
2011-07-12 23:24 ` Andrew Morton
2011-07-13 22:27 ` Hugh Dickins
2011-06-14 10:43 ` [PATCH 2/12] mm: let swap use exceptional entries Hugh Dickins
2011-06-18 21:52 ` Andrew Morton
2011-07-12 22:08 ` Hugh Dickins
2011-07-13 23:11 ` Andrew Morton
2011-07-19 22:46 ` Hugh Dickins
2011-06-18 21:55 ` Andrew Morton
2011-07-12 22:35 ` Hugh Dickins
2011-06-14 10:45 ` [PATCH 3/12] tmpfs: demolish old swap vector support Hugh Dickins
2011-06-14 10:48 ` [PATCH 4/12] tmpfs: miscellaneous trivial cleanups Hugh Dickins
2011-06-14 10:49 ` [PATCH 5/12] tmpfs: copy truncate_inode_pages_range Hugh Dickins
2011-06-14 10:51 ` [PATCH 6/12] tmpfs: convert shmem_truncate_range to radix-swap Hugh Dickins
2011-06-14 10:52 ` [PATCH 7/12] tmpfs: convert shmem_unuse_inode " Hugh Dickins
2011-06-14 10:53 ` [PATCH 8/12] tmpfs: convert shmem_getpage_gfp " Hugh Dickins
2011-06-14 10:54 ` [PATCH 9/12] tmpfs: convert mem_cgroup shmem " Hugh Dickins
2011-06-14 10:56 ` [PATCH 10/12] tmpfs: convert shmem_writepage and enable swap Hugh Dickins
2011-06-14 10:57 ` [PATCH 11/12] tmpfs: use kmemdup for short symlinks Hugh Dickins
2011-06-14 11:16 ` Pekka Enberg
2011-06-14 10:59 ` [PATCH 12/12] mm: a few small updates for radix-swap Hugh Dickins
2011-06-15 0:49 ` [PATCH v2 " Hugh Dickins
2011-06-14 17:29 ` [PATCH 0/12] tmpfs: convert from old swap vector to radix tree Linus Torvalds
2011-06-14 18:20 ` Rik van Riel
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