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

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Date: Fri, 23 Feb 2024 20:30:19 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Matthew Wilcox <willy@...radead.org>, Dan Williams
	<dan.j.williams@...el.com>
CC: Dave Hansen <dave.hansen@...el.com>, John Groves <John@...ves.net>, "John
 Groves" <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>, 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>, <linux-cxl@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<nvdimm@...ts.linux.dev>, <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

Matthew Wilcox wrote:
> On Fri, Feb 23, 2024 at 03:50:33PM -0800, Dan Williams wrote:
> > Certainly something like that would have satisified this sanity test use
> > case. I will note that mm_account_fault() would need some help to figure
> > out the size of the page table entry that got installed. Maybe
> > extensions to vm_fault_reason to add VM_FAULT_P*D? That compliments
> > VM_FAULT_FALLBACK to indicate whether, for example, the fallback went
> > from PUD to PMD, or all the way back to PTE.
> 
> ugh, no, it's more complicated than that.  look at the recent changes to
> set_ptes().  we can now install PTEs of many different sizes, depending
> on the architecture.  someday i look forward to supporting all the page
> sizes on parisc (4k, 16k, 64k, 256k, ... 4G)

Nice!

There are enough bits in vm_fault_t to represent many page sizes instead
of the entry type as I suggested, but I would defer to you or Dave on
how to make "installed pte size" generically traceable per Dave's
suggestion.