linux-kernel - Re: [PATCH v2 2/4] mm: Add file_operations.get_mapping

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Message-ID: <aTWpjOhLOMOB2e74@nvidia.com>
Date: Sun, 7 Dec 2025 12:21:32 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: kvm@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Nico Pache <npache@...hat.com>, Zi Yan <ziy@...dia.com>,
	Alex Mastro <amastro@...com>, David Hildenbrand <david@...hat.com>,
	Alex Williamson <alex@...zbot.org>, Zhi Wang <zhiw@...dia.com>,
	David Laight <david.laight.linux@...il.com>,
	Yi Liu <yi.l.liu@...el.com>, Ankit Agrawal <ankita@...dia.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/4] mm: Add file_operations.get_mapping_order()

On Thu, Dec 04, 2025 at 10:10:01AM -0500, Peter Xu wrote:
> Add one new file operation, get_mapping_order().  It can be used by file
> backends to report mapping order hints.
> 
> By default, Linux assumed we will map in PAGE_SIZE chunks.  With this hint,
> the driver can report the possibility of mapping chunks that are larger
> than PAGE_SIZE.  Then, the VA allocator will try to use that as alignment
> when allocating the VA ranges.
> 
> This is useful because when chunks to be mapped are larger than PAGE_SIZE,
> VA alignment matters and it needs to be aligned with the size of the chunk
> to be mapped.
> 
> Said that, no matter what is the alignment used for the VA allocation, the
> driver can still decide which size to map the chunks.  It is also not an
> issue if it keeps mapping in PAGE_SIZE.
> 
> get_mapping_order() is defined to take three parameters.  Besides the 1st
> parameter which will be the file object pointer, the 2nd + 3rd parameters
> being the pgoff + size of the mmap() request.  Its retval is defined as the
> order, which must be non-negative to enable the alignment.  When zero is
> returned, it should behave like when the hint is not provided, IOW,
> alignment will still be PAGE_SIZE.

This should explain how it works when the incoming pgoff is not
aligned..

I think for dpdk we want to support mapping around the MSI hole so
something like

 pgoff 0 -> 2M
 skip 4k
 2m + 4k -> 64M

Should setup the last VMA to align to 2M + 4k so the first PMD is
fragmented to 4k pages but the remaning part is 2M sized or better.

We just noticed a bug very similer to this in qemu around it's manual
alignment scheme where it would de-align things around the MSI window
and spoil the PMDs.

I guess ideally the file could return the order assuming an aligned-to-start
pgoff and the core code could use that order to compute an adjustment
for
the actual pgoff so we maintain:
  va % order = pgoff % order

Jason