linux-ext4 - Re: [RFC 00/10] fs/dax: Fix FS DAX page reference counts

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Date: Fri, 12 Apr 2024 10:32:44 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>, Alistair Popple <apopple@...dia.com>
CC: Dan Williams <dan.j.williams@...el.com>, <linux-mm@...ck.org>,
	<david@...morbit.com>, <jhubbard@...dia.com>, <rcampbell@...dia.com>,
	<willy@...radead.org>, <linux-fsdevel@...r.kernel.org>, <jack@...e.cz>,
	<djwong@...nel.org>, <hch@....de>, <david@...hat.com>,
	<ruansy.fnst@...itsu.com>, <nvdimm@...ts.linux.dev>,
	<linux-xfs@...r.kernel.org>, <linux-ext4@...r.kernel.org>,
	<jglisse@...hat.com>
Subject: Re: [RFC 00/10] fs/dax: Fix FS DAX page reference counts

Jason Gunthorpe wrote:
> On Fri, Apr 12, 2024 at 04:55:31PM +1000, Alistair Popple wrote:
> 
> > Ok, I think I found the dragons you were talking about earlier for
> > device-dax. I completely broke that because as you've already pointed
> > out pmd_trans_huge() won't filter out DAX pages. That's fine for FS DAX
> > (because the pages are essentially normal pages now anyway), but we
> > don't have a PMD equivalent of vm_normal_page() which leads to all sorts
> > of issues for DEVDAX.
> 
> What about vm_normal_page() depends on the radix level ?
> 
> Doesn't DEVDAX memory have struct page too?

Yes.

> > So I will probably have to add something like that unless we only need
> > to support large (pmd/pud) mappings of DEVDAX pages on systems with
> > CONFIG_ARCH_HAS_PTE_SPECIAL in which case I guess we could just filter
> > based on pte_special().
> 
> pte_special should only be used by memory without a struct page, is
> that what DEVDAX is?

Right, I don't think pte_special is applicable for any DAX pages.