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

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Date: Fri, 23 Feb 2024 10:23:21 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: John Groves <John@...ves.net>, John Groves <jgroves@...ron.com>,
 Jonathan Corbet <corbet@....net>, Dan Williams <dan.j.williams@...el.com>,
 Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Matthew Wilcox <willy@...radead.org>, linux-cxl@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev
Cc: john@...alactic.com, Dave Chinner <david@...morbit.com>,
 Christoph Hellwig <hch@...radead.org>, dave.hansen@...ux.intel.com,
 gregory.price@...verge.com
Subject: Re: [RFC PATCH 16/20] famfs: Add fault counters

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?