Patchwork [3/3] nfsd: vfs_llseek() with O_32BITHASH or O_64BITHASH

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Submitter Bernd Schubert
Date July 27, 2011, 11:02 a.m.
Message ID <20110727110259.204979.56782.stgit@localhost.localdomain>
Download mbox | patch
Permalink /patch/107030/
State New
Headers show

Comments

Bernd Schubert - July 27, 2011, 11:02 a.m.
Use 32-bit or 64-bit llseek() hashes for directory offsets depending on
the NFS version. NFSv2 gets 32-bit hashes only.

Signed-off-by: Bernd Schubert <bernd.schubert@itwm.fraunhofer.de>
---
 fs/nfsd/vfs.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)


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Christoph Hellwig - July 27, 2011, 9:03 p.m.
On Wed, Jul 27, 2011 at 01:02:59PM +0200, Bernd Schubert wrote:
> Use 32-bit or 64-bit llseek() hashes for directory offsets depending on
> the NFS version. NFSv2 gets 32-bit hashes only.

Independent of the O_ vs FMODE thing make sure you pass the correct
flag at open time, instead of racy runtime modifications.

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Bernd Schubert - July 28, 2011, 9:19 a.m.
On 07/27/2011 11:03 PM, Christoph Hellwig wrote:
> On Wed, Jul 27, 2011 at 01:02:59PM +0200, Bernd Schubert wrote:
>> Use 32-bit or 64-bit llseek() hashes for directory offsets depending on
>> the NFS version. NFSv2 gets 32-bit hashes only.
>
> Independent of the O_ vs FMODE thing make sure you pass the correct
> flag at open time, instead of racy runtime modifications.
>

Christoph,

before I'm going to work further on the patch sets, I have few questions 
first. Could you please help me with that?

file->f_mode is set in __dentry_open() based on O_ flags.

So if f_mode is supposed to be set directly during the NFS open call we 
would need O_ *and* FMODE flags,

Now I still do not understand why we cannot add a flag *after* the open 
call and only in nfsd_readdir()? I do not see any races there.

nfsd_readdir() gets its 'struct file' from get_empty_filp() and 
__dentry_open(). Now as 'struct file' is allocated by get_empty_filp() 
it also cannot be used by any other thread.

nfsd_readdir() just reads the directory and closes it immedeatily after 
the readdir().

So where is there supposed to be a race?


And lastly, if we are going to set f_mode directly at open time, we have 
the choice to

1) Specify those new O_ flags for all files. While setting a flag is a 
cheap operation, it still wastes CPU cycles for file opens, as files do 
not need that flag.

2) Duplicate nfsd_open() to implement a new function for directories 
only. I think not a good idea either.

3) Rewrite the nfsd_open() function to allow to set flags from calling 
functions. That would mean to update the nfsd code at at least 8 places. 
Do we really want to go that way?


So altogether, updating the patches to replace O_ by FMODE flags is 
easy, but setting that flag in nfsd at open time, would mean a large 
overhead.


Thanks,
Bernd

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Christoph Hellwig - July 28, 2011, 11:46 a.m.
On Thu, Jul 28, 2011 at 11:19:07AM +0200, Bernd Schubert wrote:
> before I'm going to work further on the patch sets, I have few
> questions first. Could you please help me with that?
> 
> file->f_mode is set in __dentry_open() based on O_ flags.

Or right after dentry_open, before anyone can possibly grab a reference
to the file.  Just do that in nfsd_open based on an NFSD_MAY_ flag.

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Patch

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index fd0acca..d79bbcd 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1994,6 +1994,12 @@  nfsd_readdir(struct svc_rqst *rqstp, struct svc_fh *fhp, loff_t *offsetp,
 	if (err)
 		goto out;
 
+	/* NFSv2 only supports 32 bit cookies */
+	if (rqstp->rq_vers > 2)
+		file->f_flags &= O_64BITHASH;
+	else
+		file->f_flags &= O_32BITHASH;
+
 	offset = vfs_llseek(file, offset, 0);
 	if (offset < 0) {
 		err = nfserrno((int)offset);