linux-kernel - Re: [PATCH v6 19/26] proc/task_mmu: Mark devdax and fsdax pages as always unpinned

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Message-ID: <6785cbb7125bf_20fa29418@dwillia2-xfh.jf.intel.com.notmuch>
Date: Mon, 13 Jan 2025 18:28:07 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Alistair Popple <apopple@...dia.com>, <akpm@...ux-foundation.org>,
	<dan.j.williams@...el.com>, <linux-mm@...ck.org>
CC: <alison.schofield@...el.com>, Alistair Popple <apopple@...dia.com>,
	<lina@...hilina.net>, <zhang.lyra@...il.com>,
	<gerald.schaefer@...ux.ibm.com>, <vishal.l.verma@...el.com>,
	<dave.jiang@...el.com>, <logang@...tatee.com>, <bhelgaas@...gle.com>,
	<jack@...e.cz>, <jgg@...pe.ca>, <catalin.marinas@....com>, <will@...nel.org>,
	<mpe@...erman.id.au>, <npiggin@...il.com>, <dave.hansen@...ux.intel.com>,
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	<tytso@....edu>, <linmiaohe@...wei.com>, <david@...hat.com>,
	<peterx@...hat.com>, <linux-doc@...r.kernel.org>,
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	<linux-ext4@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
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	<chenhuacai@...nel.org>, <kernel@...0n.name>, <loongarch@...ts.linux.dev>
Subject: Re: [PATCH v6 19/26] proc/task_mmu: Mark devdax and fsdax pages as
 always unpinned

Alistair Popple wrote:
> The procfs mmu files such as smaps and pagemap currently ignore devdax and
> fsdax pages because these pages are considered special. A future change
> will start treating these as normal pages, meaning they can be exposed via
> smaps and pagemap.
> 
> The only difference is that devdax and fsdax pages can never be pinned for
> DMA via FOLL_LONGTERM, so add an explicit check in pte_is_pinned() to
> reflect that.

I don't understand this patch.

pin_user_pages() is also used for Direct-I/O page pinning, so the
comment about FOLL_LONGTERM is wrong, and I otherwise do not understand
what goes wrong if the only pte_is_pinned() user correctly detects the
pin state?