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

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Date: Tue, 2 Jul 2024 09:18:31 +0200
From: David Hildenbrand <david@...hat.com>
To: 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
Cc: 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,
 hch@....de, david@...morbit.com
Subject: Re: [PATCH 06/13] mm/memory: Add dax_insert_pfn

On 27.06.24 02:54, Alistair Popple wrote:
> Currently to map a DAX page the DAX driver calls vmf_insert_pfn. This
> creates a special devmap PTE entry for the pfn but does not take a
> reference on the underlying struct page for the mapping. This is
> because DAX page refcounts are treated specially, as indicated by the
> presence of a devmap entry.
> 
> To allow DAX page refcounts to be managed the same as normal page
> refcounts introduce dax_insert_pfn. This will take a reference on the
> underlying page much the same as vmf_insert_page, except it also
> permits upgrading an existing mapping to be writable if
> requested/possible.

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()?

-- 
Cheers,

David / dhildenb