linux-kernel - Re: [PATCH] Revert "__d_unalias() should refuse to move mountpoints"

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Date:	Tue, 04 Dec 2012 11:37:22 +0100
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Revert "__d_unalias() should refuse to move mountpoints"

Op 04-12-12 11:33, Maarten Lankhorst schreef:
> Hey,
>
> Op 29-11-12 21:06, Al Viro schreef:
>> On Tue, Sep 25, 2012 at 04:29:58AM -0700, Eric W. Biederman wrote:
>>> Maarten Lankhorst <maarten.lankhorst@...onical.com> writes:
>>>
>>>>> Could you try the following patch?  This should report what directories
>>>>> cannot be renamed because one of them is a mount point and it gives some
>>>>> real insight into what is going on.
>>>> ls /
>>>> __d_unalias: /dev -> /dev
>>>> __d_unalias: /proc -> /proc
>>>> __d_unalias: /sys -> /sys
>>> Ok.  That is what I thought was going on.  For some reason nfs is
>>> attempting to recreate an existing dentry.
>>>
>>> Does this fix the nfs problem for you?
>>>
>>> Eric
>>>
>>> diff --git a/fs/dcache.c b/fs/dcache.c
>>> index 8086636..6390f0f 100644
>>> --- a/fs/dcache.c
>>> +++ b/fs/dcache.c
>>> @@ -2404,6 +2404,9 @@ out_unalias:
>>>  	if (likely(!d_mountpoint(alias))) {
>>>  		__d_move(alias, dentry);
>>>  		ret = alias;
>>> +	} else if ((alias->d_parent == dentry->d_parent) &&
>>> +		   !dentry_cmp(alias, dentry->d_name.name, dentry->d_name.len))
>>> +		ret = alias;
>>>  	}
>> The interesting question is why the hell had it decided that preexisting
>> dentry was not good enough for it?  Note that we have arrived to nfs_lookup()
>> after we'd decided *not* to use the damn alias.  The trace posted upthread
>> went __lookup_hash() -> lookup_real().  It means that lookup_dcache()
>> has not produced this one.  And no, even if ->d_revalidate() decided it
>> was no good, the logics in d_invalidate() would've said "busy" and we'd
>> gone with that dentry anyway.  So it means that d_lookup() has not
>> found it at all.
>>
>> IOW, something out there is blindly unhashing mountpoint dentries; that's
>> where the real root of the problem seems to be.  Could you slap
>> WARN_ON(d_mountpoint(dentry)) in __d_drop() and see what it catches?
>>
> Sorry for replying so late, I thought I wasn't hitting the bug any more, I was wrong..
>
> ------------[ cut here ]------------
> WARNING: at fs/dcache.c:452 d_drop+0x58/0x60()
> Hardware name: Aspire M3985
> Modules linked in: snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_intel snd_hda_codec snd_hwdep snd_pcm snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq kvm_intel kvm snd_timer snd_seq_device radeon snd usb_storage parport_pc ttm soundcore drm_kms_helper snd_page_alloc ppdev drm parport mei agpgart netconsole configfs nfsd
> Pid: 1497, comm: ls Not tainted 3.7.0-rc4-patser+ #517
> Call Trace:
>  [<ffffffff8104cd8f>] warn_slowpath_common+0x7f/0xc0
>  [<ffffffff8104cdea>] warn_slowpath_null+0x1a/0x20
>  [<ffffffff81187be8>] d_drop+0x58/0x60
>  [<ffffffff81256881>] nfs_readdir_page_filler+0x271/0x460
>  [<ffffffff81257e59>] nfs_readdir_xdr_to_array+0x1f9/0x2e0
>  [<ffffffff81257f66>] nfs_readdir_filler+0x26/0x90
>  [<ffffffff81119e15>] ? add_to_page_cache_lru+0x35/0x50
>  [<ffffffff8111a622>] do_read_cache_page+0x82/0x1a0
>  [<ffffffff81257f40>] ? nfs_readdir_xdr_to_array+0x2e0/0x2e0
>  [<ffffffff81183d40>] ? sys_ioctl+0xb0/0xb0
>  [<ffffffff8111a78c>] read_cache_page_async+0x1c/0x20
>  [<ffffffff8111a79e>] read_cache_page+0xe/0x20
>  [<ffffffff81258327>] nfs_readdir+0x137/0x510
>  [<ffffffff811840e1>] ? vfs_readdir+0x81/0xf0
>  [<ffffffff8126dfe0>] ? nfs3_xdr_dec_getattr3res+0x80/0x80
>  [<ffffffff81183d40>] ? sys_ioctl+0xb0/0xb0
>  [<ffffffff81184118>] vfs_readdir+0xb8/0xf0
>  [<ffffffff8118426e>] sys_getdents+0x8e/0x120
>  [<ffffffff81753394>] tracesys+0xdd/0xe2
> ---[ end trace 61d6a607ecd4e587 ]---
> ------------[ cut here ]------------
>
Using addr2line, it is indeed nfs_prime_dcache dropping the entry, in the !nfs_same_file case.

~Maarten
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