Re: [RFC PATCH 16/20] famfs: Add fault counters

John Groves
2024-02-23T19:56:45Z

On 24/02/23 10:23AM, Dave Hansen wrote:
> On 2/23/24 09:42, John Groves wrote:
> > One of the key requirements for famfs is that it service vma faults
> > efficiently. Our metadata helps - the search order is n for n extents,
> > and n is usually 1. But we can still observe gnarly lock contention
> > in mm if PTE faults are happening. This commit introduces fault counters
> > that can be enabled and read via /sys/fs/famfs/...
> >
> > These counters have proved useful in troubleshooting situations where
> > PTE faults were happening instead of PMD. No performance impact when
> > disabled.
>
> This seems kinda wonky. Why does _this_ specific filesystem need its
> own fault counters. Seems like something we'd want to do much more
> generically, if it is needed at all.
>
> Was the issue here just that vm_ops->fault() was getting called instead
> of ->huge_fault()? Or something more subtle?

Thanks for your reply Dave!

First, I'm willing to pull the fault counters out if the brain trust doesn't
like them.

I put them in because we were running benchmarks of computational data
analytics and and noted that jobs took 3x as long on famfs as raw dax -
which indicated I was doing something wrong, because it should be equivalent
or very close.

The the solution was to call thp_get_unmapped_area() in
famfs_file_operations, and performance doesn't vary significantly from raw
dax now. Prior to that I wasn't making sure the mmap address was PMD aligned.

After that I wanted a way to be double-secret-certain that it was servicing
PMD faults as intended. Which it basically always is, so far. (The smoke
tests in user space check this.)

John