using --rw doesn't work with with perf record

[sidkumar99]

I am trying to run perf record in a virtme-ng vm with --rw as a passed in option so that I can save the perf.data file onto the host file system however it fails with:

failed to write perf data, error: Bad address
failed to write perf data, error: Invalid argument

I've tried many different paths and all return this error.

[matttbe]

Hello,

It is usually not a good idea to use --rw (except if another root directory is used, e.g. from a container or with --root), hence the warning in the help menu.

Do you have the same issue with --rwdir? If yes, which versions are you using (vng, guest and host kernels, distribution, etc.) and how did you install vng?

[jremus]

I have run into the same issue:

$ vng -v --rwdir=$HOME/temp --user=root
root@virtme-ng:/home/jremus/temp# perf record -F 9999 --call-graph fp objdump -h /bin/objdump
...
[   64.953088] netfs: Couldn't get user pages (rc=-14)
failed to write perf data, error: Bad address
failed to write perf data, error: Invalid argument

A colleague of mine suggested to try /tmp, as that would be RW anyway, and that works fine:

root@virtme-ng:/tmp# perf record -F 9999 --call-graph fp objdump -h /bin/objdump
...
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.099 MB perf.data (527 samples) ]

[matttbe]

@jremus thank you for having checked that. Just to be sure, does cp /tmp/perf.data $HOME/temp/ work fine? (so OK to write in this rwdir mount?)

[jremus]

That works:

root@virtme-ng:/tmp# cp /tmp/perf.data /home/jremus/temp/
root@virtme-ng:/tmp# ls -al /home/jremus/temp
total 152
drwxr-xr-x  2 jremus jremus   4096 Oct 24 11:53 .
drwx------ 19 jremus jremus   4096 Oct 24 11:31 ..
-rw-------  1 jremus jremus 142772 Oct 24 11:52 perf.data
-rw-------  1 jremus jremus   1424 Oct 24 11:00 perf.data.old

[jremus]

Could it be that the issue is somehow caused by the --rwdir=$HOME/temp directory (e.g. /home/jremus/temp) being owned by the current user (e.g. jremus) while it is then accessed as root due to --user=root?

perf record can also write to $HOME/.debug (= /run/tmp/roothome) without any issues, where it stores debug information.

[matttbe]

Could it be that the issue is somehow caused by the --rwdir=$HOME/temp directory (e.g. /home/jremus/temp) being owned by the current user (e.g. jremus) while it is then accessed as root due to --user=root?

@jremus: maybe? It is just strange that you can still write the file from the same user, but perf can't. With --rwdir, 9p is used, so it should not be an issue with virtiofsd. With 9p, it should write the files as the user which started the VM I think, so it is probably normal.

I tried to reproduce it using my host kernel (v6.17), perf (v6.17), and the dev version of vng and everything is OK for me. perf can write the file directly in $PWD/temp, but it outputs the following:

perf record -F 9999 --call-graph fp objdump -h /bin/objdump
(...)
[ perf record: Woken up 1 times to write data ]                                                                         
failed to mmap file                                                                                                     
[ perf record: Captured and wrote 0.004 MB perf.data ]

Probably it is not possible to use mmap, and in newer perf, it fallbacks to something else?