From: "Tobin C. Harding" <me@tobin.cc>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
"Tobin C. Harding" <tobin@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Roman Gushchin <guro@fb.com>,
Alexander Viro <viro@ftp.linux.org.uk>,
Christoph Hellwig <hch@infradead.org>,
Pekka Enberg <penberg@cs.helsinki.fi>,
David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Christopher Lameter <cl@linux.com>,
Matthew Wilcox <willy@infradead.org>,
Miklos Szeredi <mszeredi@redhat.com>,
Andreas Dilger <adilger@dilger.ca>,
Waiman Long <longman@redhat.com>, Tycho Andersen <tycho@tycho.ws>,
Theodore Ts'o <tytso@mit.edu>, Andi Kleen <ak@linux.intel.com>,
David Chinner <david@fromorbit.com>,
Nick Piggin <npiggin@gmail.com>, Rik van Riel <riel@redhat.com>,
Hugh Dickins <hughd@google.com>, Jonathan Corbet <corbet@lwn.net>,
linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: shrink_dentry_list() logics change (was Re: [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab Movable Objects)
Date: Mon, 1 Jul 2019 19:26:25 +1000 [thread overview]
Message-ID: <20190701092625.GA9703@ares> (raw)
In-Reply-To: <20190629190624.GU17978@ZenIV.linux.org.uk>
On Sat, Jun 29, 2019 at 08:06:24PM +0100, Al Viro wrote:
> On Sat, Jun 29, 2019 at 05:38:03AM +0100, Al Viro wrote:
>
> > PS: the problem is not gone in the next iteration of the patchset in
> > question. The patch I'm proposing (including dput_to_list() and _ONLY_
> > compile-tested) follows. Comments?
>
> FWIW, there's another unpleasantness in the whole thing. Suppose we have
> picked a page full of dentries, all with refcount 0. We decide to
> evict all of them. As it turns out, they are from two filesystems.
> Filesystem 1 is NFS on a server, with currently downed hub on the way
> to it. Filesystem 2 is local. We attempt to evict an NFS dentry and
> get stuck - tons of dirty data with no way to flush them on server.
> In the meanwhile, admin tries to unmount the local filesystem. And
> gets stuck as well, since umount can't do anything to its dentries
> that happen to sit in our shrink list.
>
> I wonder if the root of problem here isn't in shrink_dcache_for_umount();
> all it really needs is to have everything on that fs with refcount 0
> dragged through __dentry_kill(). If something had been on a shrink
> list, __dentry_kill() will just leave behind a struct dentry completely
> devoid of any connection to superblock, other dentries, filesystem
> type, etc. - it's just a piece of memory that won't be freed until
> the owner of shrink list finally gets around to it. Which can happen
> at any point - all they'll do to it is dentry_free(), and that doesn't
> need any fs-related data structures.
>
> The logics in shrink_dcache_parent() is
> collect everything evictable into a shrink list
> if anything found - kick it out and repeat the scan
> otherwise, if something had been on other's shrink list
> repeat the scan
>
> I wonder if after the "no evictable candidates, but something
> on other's shrink lists" we ought to do something along the
> lines of
> rcu_read_lock
> walk it, doing
> if dentry has zero refcount
> if it's not on a shrink list,
> move it to ours
> else
> store its address in 'victim'
> end the walk
> if no victim found
> rcu_read_unlock
> else
> lock victim for __dentry_kill
> rcu_read_unlock
> if it's still alive
> if it's not IS_ROOT
> if parent is not on shrink list
> decrement parent's refcount
> put it on our list
> else
> decrement parent's refcount
> __dentry_kill(victim)
> else
> unlock
> if our list is non-empty
> shrink_dentry_list on it
> in there...
Thanks for still thinking about this Al. I don't have a lot of idea
about what to do with your comments until I can grok them fully but I
wanted to acknowledge having read them.
Thanks,
Tobin.
next prev parent reply other threads:[~2019-07-01 9:26 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-11 1:34 [RFC PATCH v3 00/15] Slab Movable Objects (SMO) Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 01/15] slub: Add isolate() and migrate() methods Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 02/15] tools/vm/slabinfo: Add support for -C and -M options Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 03/15] slub: Sort slab cache list Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 04/15] slub: Slab defrag core Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 05/15] tools/vm/slabinfo: Add remote node defrag ratio output Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 06/15] tools/vm/slabinfo: Add defrag_used_ratio output Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 07/15] tools/testing/slab: Add object migration test module Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 08/15] tools/testing/slab: Add object migration test suite Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 09/15] xarray: Implement migration function for objects Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 10/15] tools/testing/slab: Add XArray movable objects tests Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 11/15] slub: Enable moving objects to/from specific nodes Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 12/15] slub: Enable balancing slabs across nodes Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 13/15] dcache: Provide a dentry constructor Tobin C. Harding
2019-04-11 1:34 ` [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab Movable Objects Tobin C. Harding
2019-04-11 2:33 ` Al Viro
2019-04-11 2:48 ` Tobin C. Harding
2019-04-11 4:47 ` Al Viro
2019-04-11 5:05 ` Tobin C. Harding
2019-04-11 20:01 ` Al Viro
2019-04-11 21:02 ` Al Viro
2019-06-29 4:08 ` Al Viro
2019-06-29 4:38 ` shrink_dentry_list() logics change (was Re: [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab Movable Objects) Al Viro
2019-06-29 19:06 ` Al Viro
2019-06-29 22:29 ` Al Viro
2019-06-29 22:34 ` Al Viro
2019-07-01 9:26 ` Tobin C. Harding [this message]
2019-04-11 1:34 ` [RFC PATCH v3 15/15] dcache: Add CONFIG_DCACHE_SMO Tobin C. Harding
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