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

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Message-ID: <87zfr0f4oo.fsf@nvdebian.thelocal>
Date: Tue, 02 Jul 2024 20:47:50 +1000
From: Alistair Popple <apopple@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: dan.j.williams@...el.com, vishal.l.verma@...el.com,
 dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com,
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 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
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 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


David Hildenbrand <david@...hat.com> writes:

> 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()?

It's not something I've looked at but quite possibly - there are a
couple of other users of vmf_insert_mixed() that would need to be
removed though. I just added this as dax_insert_pfn() because as an API
it is really specific to the DAX use case. For example a more general
API would likely pass a page/folio.