Hi Al Viro,
Às 20:19 de 23/03/21, Al Viro escreveu:
On Tue, Mar 23, 2021 at 04:59:39PM -0300, André Almeida wrote:
* dcache handling:
For now, negative lookups are not inserted in the dcache, since they
would need to be invalidated anyway, because we can't trust missing file
dentries. This is bad for performance but requires some leveraging of
the VFS layer to fix. We can live without that for now, and so does
everyone else.
"For now"? Not a single practical suggestion has ever materialized.
Pardon me, but by now I'm very sceptical about the odds of that
ever changing. And no, I don't have any suggestions either.
Right, I'll reword this to reflect that there's no expectation that this
will be done, while keeping documented this performance issue.
The lookup() path at tmpfs creates negatives dentries, that are later
instantiated if the file is created. In that way, all files in tmpfs
have a dentry given that the filesystem exists exclusively in memory.
As explained above, we don't have negative dentries for casefold files,
so dentries are created at lookup() iff files aren't casefolded. Else,
the dentry is created just before being instantiated at create path.
At the remove path, dentries are invalidated for casefolded files.
Umm... What happens to those assertions if previously sane directory
gets case-buggered? You've got an ioctl for doing just that...
Incidentally, that ioctl is obviously racy - result of that simple_empty()
might have nothing to do with reality before it is returned to caller.
And while we are at it, simple_empty() doesn't check a damn thing about
negative dentries in there...
Thanks for pointing those issues. I'll move my lock at IOCTL to make
impossible to change directory attributes and add a file there at the
same time. About the negative dentries that existed before at that
directory, I believe the way to solve this is by invalidating them all.
How that sound to you?
Thanks,
André
