Re: [PATCH v3] fs: try an opportunistic lookup for O_CREAT opens too

[Josef Bacik]

On Wed, Aug 07, 2024 at 08:10:27AM -0400, Jeff Layton wrote:
> Today, when opening a file we'll typically do a fast lookup, but if
> O_CREAT is set, the kernel always takes the exclusive inode lock. I
> assume this was done with the expectation that O_CREAT means that we
> always expect to do the create, but that's often not the case. Many
> programs set O_CREAT even in scenarios where the file already exists.
> 
> This patch rearranges the pathwalk-for-open code to also attempt a
> fast_lookup in certain O_CREAT cases. If a positive dentry is found, the
> inode_lock can be avoided altogether, and if auditing isn't enabled, it
> can stay in rcuwalk mode for the last step_into.
> 
> One notable exception that is hopefully temporary: if we're doing an
> rcuwalk and auditing is enabled, skip the lookup_fast. Legitimizing the
> dentry in that case is more expensive than taking the i_rwsem for now.
> 
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>

Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>

Thanks,

Josef