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From: Andreas Dilger <adilger@dilger.ca>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: NeilBrown <neilb@suse.de>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Oleg Drokin <oleg.drokin@intel.com>,
	Andreas Dilger <andreas.dilger@intel.com>
Subject: Re: [PATCH 02/20] STAGING/lustre: limit follow_link recursion using stack space.
Date: Sun, 19 Apr 2015 14:57:07 -0600	[thread overview]
Message-ID: <AE16A656-74E1-468B-A1B6-4D30EB05B8A7@dilger.ca> (raw)
In-Reply-To: <20150418030128.GF889@ZenIV.linux.org.uk>

On Apr 17, 2015, at 9:01 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> 
> On Mon, Mar 23, 2015 at 01:37:38PM +1100, NeilBrown wrote:
>> lustre's ->follow_link() uses a lot of stack space and so
>> need to limit symlink recursion based on stack size.
>> 
>> It currently tests current->link_count, but that will soon
>> become private to fs/namei.c.
>> So instead base on actual available stack space.
>> This patch aborts recursive symlinks in less than 2K of space
>> is available.  This seems consistent with current code, but
>> hasn't been tested.
> 
> BTW, in the best case that logics is fishy.  We have "up to 5 levels with
> 4Kb stack and up to 7 with 8Kb one".  Could somebody manage to dig out
> the reasons for such limits?  Preferably along with the kernel version
> where the overflows had been observed, both for 4K and 8K cases.

Hi Al,
I checked in our bug history, and the 8KB stack limit was hit with
older clients running racer or our recursive-symlink regression test:

2.6.18: https://bugzilla.lustre.org/show_bug.cgi?id=18533#c0
2.6.16: https://bugzilla.lustre.org/show_bug.cgi?id=19380#c11

The 4KB stack limit for clients has existed a lot longer than that,
but CONFIG_4KSTACKS was not the default on all kernels for a while.
The following bug showed a stack overflow with 2.6.22 kernels:

https://bugzilla.lustre.org/show_bug.cgi?id=17379#c0

Prior to 2.6.16 when we needed client-side kernel patches and a custom
kernel build, we always forced the CONFIG_4KSTACKS off in the config.

In general, Lustre is a heavy stack user because it is a network
filesystem, and doubly so if the Lustre client is re-exporting the
filesystem to NFS clients.

I'd be happy if symlink recursion was removed completely, but so far the
added symlink recursion limit hasn't been a problem for Lustre users.

Cheers, Andreas






  reply	other threads:[~2015-04-19 20:57 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-23  2:37 [PATCH 00/20] Support follow_link in RCU-walk - V3 NeilBrown
2015-03-23  2:37 ` [PATCH 02/20] STAGING/lustre: limit follow_link recursion using stack space NeilBrown
2015-04-18  3:01   ` Al Viro
2015-04-19 20:57     ` Andreas Dilger [this message]
2015-04-19 21:33       ` Al Viro
2015-04-20  2:29         ` Al Viro
2015-03-23  2:37 ` [PATCH 03/20] VFS: replace {, total_}link_count in task_struct with pointer to nameidata NeilBrown
2015-03-23  2:37 ` [PATCH 01/20] Documentation: remove outdated information from automount-support.txt NeilBrown
2015-03-23  2:37 ` [PATCH 10/20] security: make inode_follow_link RCU-walk aware NeilBrown
2015-03-23  2:37 ` [PATCH 04/20] ovl: rearrange ovl_follow_link to it doesn't need to call ->put_link NeilBrown
2015-03-23  2:37 ` [PATCH 07/20] VFS: remove nameidata args from ->follow_link NeilBrown
2015-03-23  2:37 ` [PATCH 06/20] SECURITY: remove nameidata arg from inode_follow_link NeilBrown
2015-03-23  2:37 ` [PATCH 05/20] VFS: replace nameidata arg to ->put_link with a char* NeilBrown
2015-03-23  2:37 ` [PATCH 09/20] security/selinux: pass 'flags' arg to avc_audit() and avc_has_perm_flags() NeilBrown
2015-03-23  2:37 ` [PATCH 11/20] VFS/namei: use terminate_walk when symlink lookup fails NeilBrown
2015-03-23  2:37 ` [PATCH 08/20] VFS: make all ->follow_link handlers aware for LOOKUP_RCU NeilBrown
2015-03-23  2:37 ` [PATCH 13/20] VFS/namei: abort RCU-walk on symlink if atime needs updating NeilBrown
2015-03-23  2:37 ` [PATCH 14/20] VFS/namei: add 'inode' arg to put_link() NeilBrown
2015-04-17 16:25   ` Al Viro
2015-04-17 19:09     ` Al Viro
2015-04-18  8:09       ` Al Viro
2015-03-23  2:37 ` [PATCH 16/20] VFS/namei: enable RCU-walk when following symlinks NeilBrown
2015-03-23  2:37 ` [PATCH 19/20] XFS: allow follow_link to often succeed in RCU-walk NeilBrown
2015-03-23  2:37 ` [PATCH 20/20] NFS: support LOOKUP_RCU in nfs_follow_link NeilBrown
2015-03-23  2:37 ` [PATCH 18/20] xfs: use RCU to free 'struct xfs_mount' NeilBrown
2015-03-23  2:37 ` [PATCH 17/20] VFS/namei: handle LOOKUP_RCU in page_follow_link_light NeilBrown
2015-03-23  2:37 ` [PATCH 12/20] VFS/namei: new flag to support RCU symlinks: LOOKUP_LINK_RCU NeilBrown
2015-03-23  2:37 ` [PATCH 15/20] VFS/namei: enhance follow_link to support RCU-walk NeilBrown
2015-03-25 23:23 ` [PATCH 00/20] Support follow_link in RCU-walk - V3 NeilBrown

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