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Date: Fri, 23 Feb 2024 20:30:19 -0800
From: Dan Williams <dan.j.williams@intel.com>
To: Matthew Wilcox <willy@infradead.org>, Dan Williams
	<dan.j.williams@intel.com>
CC: Dave Hansen <dave.hansen@intel.com>, John Groves <John@groves.net>, "John
 Groves" <jgroves@micron.com>, Jonathan Corbet <corbet@lwn.net>, Vishal Verma
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	<david@fromorbit.com>, Christoph Hellwig <hch@infradead.org>,
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Subject: Re: [RFC PATCH 16/20] famfs: Add fault counters
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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.