This is an optimization that will prevent needless lookups on lower layers for
directories. This is analogous to the Linux commit 97c684cc9110 ("ovl: create
directories inside merged parent opaque").
If mkdir(2) does succeed, then none of the lower layers have a file at that
position, otherwise mkdir(2) would have failed with EEXIST.
Changes on the lower layer underneath a new upper layer directory are not
visible. See the following for example:
```
$ sudo mount -t overlay -o lowerdir=lower,upperdir=upper,workdir=workdir none overlay
$ mkdir -p overlay/dir/dir1
$ mkdir -p lower/dir/dir2
$ ls overlay/dir/
dir1
```
Benchmarking shows that this change cuts ABSL build time by 9.5%!
```
goos: linux
goarch: amd64
cpu: Intel(R) Xeon(R) CPU @ 2.20GHz
│ /tmp/benchout.runsc │ /tmp/benchout.runsc-opt │
│ sec/op │ sec/op vs base │
BuildABSL/page_cache.clean/filesystem.bindfs-8 71.16 ± 12% 64.40 ± 3% -9.50% (p=0.003 n=8)
BuildGRPC/page_cache.clean/filesystem.bindfs-8 418.2 ± 1% 419.9 ± 1% ~ (p=0.721 n=8)
RubySpecTest/page_cache.clean/filesystem.bindfs-8 76.67 ± 5% 75.17 ± 1% ~ (p=0.382 n=8)
geomean 131.6 126.7 -3.78%
│ /tmp/benchout.runsc │ /tmp/benchout.runsc-opt │
│ load.sec │ load.sec vs base │
RubySpecTest/page_cache.clean/filesystem.bindfs-8 12.95 ± 6% 12.80 ± 5% ~ (p=0.625 n=8)
```
PiperOrigin-RevId: 538823897
These operations require CAP_SYS_ADMIN in the root user namespace. There's no
easy way to check that other than trying the operation and seeing what happens.
PiperOrigin-RevId: 331242256
As in VFS1, we only support the user.* namespace. Plumbing is added to tmpfs
and goferfs.
Note that because of the slightly different order of checks between VFS2 and
Linux, one of the xattr tests needs to be relaxed slightly.
Fixes#2363.
PiperOrigin-RevId: 305985121
Note that these simply will use the same logic as getxattr and setxattr, which
is not yet implemented for most filesystems.
PiperOrigin-RevId: 290800960
These can remain disabled until we actually support extended attributes.
The following modifications were also made:
1. Disable save/restore on tests that change file permissions. Restore will not
work properly for these tests, since it will try to open the file with
read-write after it has been read- or write-only.
2. Change user.abc to user.test.
PiperOrigin-RevId: 290123941
Add checks for input arguments, file type, permissions, etc. that match
the Linux implementation. A call to get/setxattr that passes all the
checks will still currently return EOPNOTSUPP. Actual support will be
added in following commits.
Only allow user.* extended attributes for the time being.
PiperOrigin-RevId: 285835159
