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

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Message-ID: <86efded9-c1d9-4efb-bf41-f67faa49bf69@redhat.com>
Date: Tue, 14 Jan 2025 17:45:46 +0100
From: David Hildenbrand <david@...hat.com>
To: Dan Williams <dan.j.williams@...el.com>,
 Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: alison.schofield@...el.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,
 ira.weiny@...el.com, willy@...radead.org, djwong@...nel.org, tytso@....edu,
 linmiaohe@...wei.com, peterx@...hat.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, nvdimm@...ts.linux.dev,
 linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, jhubbard@...dia.com,
 hch@....de, david@...morbit.com, 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

On 14.01.25 03:28, Dan Williams wrote:
> 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.


This whole pte_is_pinned() should likely be ripped out (and I have a 
patch here to do that for a long time).

But that's a different discussion.

> 
> 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?

Yes, this patch should likely just be dropped.

Even if folio_maybe_dma_pinned() == true because of "false positives", 
it will behave just like other order-0 pages with false positives, and 
only affect soft-dirty tracking ... which nobody should be caring about 
here at all.

We would always detect the PTE as soft-dirty because we we never
	pte_wrprotect(old_pte)

Yes, nobody should care.

-- 
Cheers,

David / dhildenb