Re: [vfs] 8bb3c61baf: vm-scalability.median -23.7% regression

From: Al Viro
Date: Sun Sep 08 2019 - 17:48:40 EST

On Tue, Sep 03, 2019 at 04:41:22PM +0800, kernel test robot wrote:
> Greeting,
> FYI, we noticed a -23.7% regression of vm-scalability.median due to commit:
> commit: 8bb3c61bafa8c1cd222ada602bb94ff23119e738 ("vfs: Convert ramfs, shmem, tmpfs, devtmpfs, rootfs to use the new mount API")
> work.mount
> in testcase: vm-scalability
> on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 128G memory
> with following parameters:
> runtime: 300s
> size: 16G
> test: shm-pread-rand
> cpufreq_governor: performance
> ucode: 0xb000036

That thing loses size=... option. Both size= and nr_blocks= affect the
same thing (->max_blocks), but the parser keeps track of the options
it has seen and applying the parsed data to superblock checks only
whether nr_blocks= had been there. IOW, size= gets parsed, but the
result goes nowhere.

I'm not sure whether it's better to fix the patch up or redo it from
scratch - it needs to be carved up anyway and it's highly non-transparent,
so I'm probably going to replace the damn thing entirely with something
that would be easier to follow.