linux-kernel - Re: [PATCH 06/13] mm/memory: Add dax_insert

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Date: Tue, 2 Jul 2024 13:53:13 +0200
From: David Hildenbrand <david@...hat.com>
To: Christoph Hellwig <hch@....de>
Cc: Alistair Popple <apopple@...dia.com>, dan.j.williams@...el.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-mm@...ck.org,
 linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, jhubbard@...dia.com,
 david@...morbit.com
Subject: Re: [PATCH 06/13] mm/memory: Add dax_insert_pfn

On 02.07.24 13:46, Christoph Hellwig wrote:
> On Tue, Jul 02, 2024 at 09:18:31AM +0200, David Hildenbrand wrote:
>> We have this comparably nasty vmf_insert_mixed() that FS dax abused to
>> insert into !VM_MIXED VMAs. Is that abuse now stopping and are there maybe
>> ways to get rid of vmf_insert_mixed()?
> 
> Unfortunately it is also used by a few drm drivers and not just DAX.

At least they all seem to set VM_MIXED:

* fs/cramfs/inode.c does
* drivers/gpu/drm/gma500/fbdev.c does
* drivers/gpu/drm/omapdrm/omap_gem.c does

Only DAX (including drivers/dax/device.c) doesn't.

VM_MIXEDMAP handling for DAX was changed in

commit e1fb4a0864958fac2fb1b23f9f4562a9f90e3e8f
Author: Dave Jiang <dave.jiang@...el.com>
Date:   Fri Aug 17 15:43:40 2018 -0700

     dax: remove VM_MIXEDMAP for fsdax and device dax

After prepared by

commit 785a3fab4adbf91b2189c928a59ae219c54ba95e
Author: Dan Williams <dan.j.williams@...el.com>
Date:   Mon Oct 23 07:20:00 2017 -0700

     mm, dax: introduce pfn_t_special()

     In support of removing the VM_MIXEDMAP indication from DAX VMAs,
     introduce pfn_t_special() for drivers to indicate that _PAGE_SPECIAL
     should be used for DAX ptes.

I wonder if there are ways forward to either remove vmf_insert_mixed() 
or at least require it (as the name suggests) to have VM_MIXEDMAP set.

-- 
Cheers,

David / dhildenb