linux-kernel - Re: backing up ext4 fs, system unresponsive, thrashing like crazy even though swap is unused

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Date:	Mon, 3 Dec 2012 21:35:00 +0200 (EET)
From:	Dimitrios Apostolou <jimis@....net>
To:	Eric Paris <eparis@...isplace.org>, Theodore Ts'o <tytso@....edu>
cc:	Roland Eggner <edvx1@...temanalysen.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>
Subject: Re: backing up ext4 fs, system unresponsive, thrashing like crazy
 even though swap is unused

On Mon, 3 Dec 2012, Eric Paris wrote:

> On Mon, Dec 3, 2012 at 12:43 PM, Theodore Ts'o <tytso@....edu> wrote:
>
>> If you are seeing a large number of inodes still in the ext4 inode
>> cache after using drop_caches, then I'd look to see whether you have
>> something like SELinux or auditing enabled which is pinning a bunch of
>> dentries or inodes
>
> You can safely ignore this suggestion as it does make sense.  SELinux
> only grabs a references to dentries during its call to
> fs_ops->getxattr, which can't last a meaningful length of time (unless
> the filesystem is busted).  It only grabs references to inodes during
> system initialization, when you couldn't have many in core.
>
> Audit, likewise, only grabs a reference to a dentry during execve()
> and only long enough to run getxattr and does not grab any reference
> directly to an inode at all.
>

AFAICT I use neither SELinux nor audit.

>> or whether your backup program (or some other
>> program running on your system) is keeping lots of directories or
>> inodes open for some reason.
>
> Certainly could be this suggestion though..
>

I've managed to reproduce the scenario with concurrent "du" commands 
running on the filesystems. I'll try doing it once more, but it may take a 
while to get the dmesg/slabinfo etc output, since even a realtime root 
shell is non-responsive for many minutes.

What other debug output do you suggest to get, to find out why ext4 
inodes are pinned?


Thanks,
Dimitris

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