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From: Charles Hedrick <hedrick@rutgers.edu>
To: "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: optimization for nfsd_set_fh_dentry
Thread-Topic: optimization for nfsd_set_fh_dentry
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Date: Sun, 7 Apr 2024 13:25:33 +0000
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nfsd_set_fh_dentry is used in setfh and a number of other places. It calls =
exportfs_decode_fh_raw with an argument nfsd_acceptable.=0A=
=0A=
That is a test that always returns 1 if NFSEXP_NOSUBTREECHECK is set.=0A=
=0A=
However there is an optimization in exportfs_decode_fh_raw that skips a lot=
 of file system stuff if NULL is passed as the test. A test that always ret=
urns 1 still triggers all the file system stuff, even though it's not neede=
d. =0A=
=0A=
nfsd_set_fh_dentry should pass NULL to exportfs_decode_fh_raw if NFSEXP_NOS=
UBTREECHECK is set.=0A=
=0A=
I discovered this while looking into a performance problem caused by a path=
ological client that was sending 40,000 RPCs/sec, almost all get =0A=
GETATTR or ACCESS. It was causing the nfsd's to be stuck at 100% CPU by loc=
ks in file system code invoked by this unnecessary code.=0A=
=0A=
(We fixed the client by mounting with NFS3 rather than NFS4. This suggests =
an issue with the NFS 4 client side. Unfortunately I don't have enough info=
rmation to make a useful report.)=0A=
=0A=
=0A=
=0A=