linux-kernel - Re: [PATCH 3/5] mm: Rename __thp_get_unmapped_area to mm_get_unmapped_area

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Message-ID: <20250613160020.GM1174925@nvidia.com>
Date: Fri, 13 Jun 2025 13:00:20 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, kvm@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Zi Yan <ziy@...dia.com>, Alex Mastro <amastro@...com>,
	David Hildenbrand <david@...hat.com>,
	Nico Pache <npache@...hat.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
	Barry Song <baohua@...nel.org>
Subject: Re: [PATCH 3/5] mm: Rename __thp_get_unmapped_area to
 mm_get_unmapped_area_aligned

On Fri, Jun 13, 2025 at 11:13:58AM -0400, Peter Xu wrote:
> > I didn't intuitively guess how it works or why there are two
> > length/size arguments. It seems to have an exciting return code as
> > well.
> > 
> > I suppose size is the alignment target? Maybe rename the parameter too?
> 
> Yes, when the kdoc is there it'll be more obvious.  So far "size" is ok to
> me, but if you have better suggestion please shoot - whatever I came up
> with so far seems to be too long, and maybe not necessary when kdoc will be
> available too.

I would call it align not size

> > For the purposes of VFIO do we need to be careful about math overflow here:
> > 
> > 	loff_t off_end = off + len;
> > 	loff_t off_align = round_up(off, size);
> > 
> > ?
> 
> IIUC the 1st one was covered by the latter check here:
> 
>         (off + len_pad) < off
> 
> Indeed I didn't see what makes sure the 2nd won't overflow.

I'm not sure the < tests are safe in this modern world. I would use
the overflow helpers directly and remove the < overflow checks.

> +/**
> + * mm_get_unmapped_area_aligned - Allocate an aligned virtual address
> + * @filp: file target of the mmap() request
> + * @addr: hint address from mmap() request
> + * @len: len of the mmap() request
> + * @off: file offset of the mmap() request
> + * @flags: flags of the mmap() request
> + * @size: the size of alignment the caller requests

Just "the alignment the caller requests"

> + * @vm_flags: the vm_flags passed from get_unmapped_area() caller
> + *
> + * This function should normally be used by a driver's specific
> + * get_unmapped_area() handler to provide a properly aligned virtual
> + * address for a specific mmap() request.  The caller should pass in most
> + * of the parameters from the get_unmapped_area() request, but properly
> + * specify @size as the alignment needed.

 .. "The function willl try to return a VMA starting address such that
 ret % size == 0"

Jason