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author | Nick Piggin <npiggin@kernel.dk> | 2011-01-07 07:49:49 +0100 |
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committer | Nick Piggin <npiggin@kernel.dk> | 2011-01-07 07:50:26 +0100 |
commit | fa0d7e3de6d6fc5004ad9dea0dd6b286af8f03e9 (patch) | |
tree | 203e0f73883e4c26b5597e36042386a1237dab35 /Documentation | |
parent | fs: consolidate dentry kill sequence (diff) | |
download | linux-fa0d7e3de6d6fc5004ad9dea0dd6b286af8f03e9.tar.xz linux-fa0d7e3de6d6fc5004ad9dea0dd6b286af8f03e9.zip |
fs: icache RCU free inodes
RCU free the struct inode. This will allow:
- Subsequent store-free path walking patch. The inode must be consulted for
permissions when walking, so an RCU inode reference is a must.
- sb_inode_list_lock to be moved inside i_lock because sb list walkers who want
to take i_lock no longer need to take sb_inode_list_lock to walk the list in
the first place. This will simplify and optimize locking.
- Could remove some nested trylock loops in dcache code
- Could potentially simplify things a bit in VM land. Do not need to take the
page lock to follow page->mapping.
The downsides of this is the performance cost of using RCU. In a simple
creat/unlink microbenchmark, performance drops by about 10% due to inability to
reuse cache-hot slab objects. As iterations increase and RCU freeing starts
kicking over, this increases to about 20%.
In cases where inode lifetimes are longer (ie. many inodes may be allocated
during the average life span of a single inode), a lot of this cache reuse is
not applicable, so the regression caused by this patch is smaller.
The cache-hot regression could largely be avoided by using SLAB_DESTROY_BY_RCU,
however this adds some complexity to list walking and store-free path walking,
so I prefer to implement this at a later date, if it is shown to be a win in
real situations. I haven't found a regression in any non-micro benchmark so I
doubt it will be a problem.
Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Diffstat (limited to 'Documentation')
-rw-r--r-- | Documentation/filesystems/porting | 14 |
1 files changed, 14 insertions, 0 deletions
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting index 1eb76959d096..ccf0ce7866b9 100644 --- a/Documentation/filesystems/porting +++ b/Documentation/filesystems/porting @@ -346,3 +346,17 @@ look at examples of other filesystems) for guidance. for details of what locks to replace dcache_lock with in order to protect particular things. Most of the time, a filesystem only needs ->d_lock, which protects *all* the dcache state of a given dentry. + +-- +[mandatory] + + Filesystems must RCU-free their inodes, if they can have been accessed +via rcu-walk path walk (basically, if the file can have had a path name in the +vfs namespace). + + i_dentry and i_rcu share storage in a union, and the vfs expects +i_dentry to be reinitialized before it is freed, so an: + + INIT_LIST_HEAD(&inode->i_dentry); + +must be done in the RCU callback. |