Date: Thu, 19 Jan 2012 16:06:13 +0200 From: Kostik Belousov <kostikbel@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: Rick Macklem <rmacklem@freebsd.org>, fs@freebsd.org, Peter Wemm <peter@freebsd.org> Subject: Re: Race in NFS lookup can result in stale namecache entries Message-ID: <20120119140613.GD31224@deviant.kiev.zoral.com.ua> In-Reply-To: <201201181707.21293.jhb@freebsd.org> References: <201201181707.21293.jhb@freebsd.org>
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--VetWPALTXTOCTxkp Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Jan 18, 2012 at 05:07:21PM -0500, John Baldwin wrote: =2E.. > What I concluded is that it would really be far simpler and more > obvious if the cached timestamps were stored in the namecache entry > directly rather than having multiple name cache entries validated by > shared state in the nfsnode. This does mean allowing the name cache > to hold some filesystem-specific state. However, I felt this was much > cleaner than adding a lot more complexity to nfs_lookup(). Also, this > turns out to be fairly non-invasive to implement since nfs_lookup() > calls cache_lookup() directly, but other filesystems only call it > indirectly via vfs_cache_lookup(). I considered letting filesystems > store a void * cookie in the name cache entry and having them provide > a destructor, etc. However, that would require extra allocations for > NFS lookups. Instead, I just adjusted the name cache API to > explicitly allow the filesystem to store a single timestamp in a name > cache entry by adding a new 'cache_enter_time()' that accepts a struct > timespec that is copied into the entry. 'cache_enter_time()' also > saves the current value of 'ticks' in the entry. 'cache_lookup()' is > modified to add two new arguments used to return the timespec and > ticks value used for a namecache entry when a hit in the cache occurs. >=20 > One wrinkle with this is that the name cache does not create actual > entries for ".", and thus it would not store any timestamps for those > lookups. To fix this I changed the NFS client to explicitly fast-path > lookups of "." by always returning the current directory as setup by > cache_lookup() and never bothering to do a LOOKUP or check for stale > attributes in that case. >=20 > The current patch against 8 is at > http://www.FreeBSD.org/~jhb/patches/nfs_lookup.patch =2E.. So now you add 8*2+4 bytes to each namecache entry on amd64 unconditionally. Current size of the struct namecache invariant part on amd64 is 72 bytes, so addition of 20 bytes looks slightly excessive. I am not sure about typical distribution of the namecache nc_name length, so it is unobvious does the change changes the memory usage significantly. A flag could be added to nc_flags to indicate the presence of timestamp. The timestamps would be conditionally placed after nc_nlen, we probably could use union to ease the access. Then, the direct dereferences of nc_name would need to be converted to some inline function. I can do this after your patch is committed, if you consider the memory usage saving worth it. --VetWPALTXTOCTxkp Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (FreeBSD) iEYEARECAAYFAk8YI1UACgkQC3+MBN1Mb4gaBgCeM1EsgbmWanasw8Mk4UO03o6J oikAnikR7N6x4S9ePHlDOrYNc0u2ihqc =7esr -----END PGP SIGNATURE----- --VetWPALTXTOCTxkp--
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