netdev - Re: [PATCH RFC 21/35] mm/cma: refuse handing out non-contiguous page ranges

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Message-ID: <aK2yhtQ0M_0hqQHh@raptor>
Date: Tue, 26 Aug 2025 14:11:34 +0100
From: Alexandru Elisei <alexandru.elisei@....com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
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Subject: Re: [PATCH RFC 21/35] mm/cma: refuse handing out non-contiguous page
 ranges

Hi David,

On Tue, Aug 26, 2025 at 03:08:08PM +0200, David Hildenbrand wrote:
> On 26.08.25 15:03, Alexandru Elisei wrote:
> > Hi David,
> > 
> > On Tue, Aug 26, 2025 at 01:04:33PM +0200, David Hildenbrand wrote:
> > ..
> > > > Just so I can better understand the problem being fixed, I guess you can have
> > > > two consecutive pfns with non-consecutive associated struct page if you have two
> > > > adjacent memory sections spanning the same physical memory region, is that
> > > > correct?
> > > 
> > > Exactly. Essentially on SPARSEMEM without SPARSEMEM_VMEMMAP it is not
> > > guaranteed that
> > > 
> > > 	pfn_to_page(pfn + 1) == pfn_to_page(pfn) + 1
> > > 
> > > when we cross memory section boundaries.
> > > 
> > > It can be the case for early boot memory if we allocated consecutive areas
> > > from memblock when allocating the memmap (struct pages) per memory section,
> > > but it's not guaranteed.
> > 
> > Thank you for the explanation, but I'm a bit confused by the last paragraph. I
> > think what you're saying is that we can also have the reverse problem, where
> > consecutive struct page * represent non-consecutive pfns, because memmap
> > allocations happened to return consecutive virtual addresses, is that right?
> 
> Exactly, that's something we have to deal with elsewhere [1]. For this code,
> it's not a problem because we always allocate a contiguous PFN range.
> 
> > 
> > If that's correct, I don't think that's the case for CMA, which deals out
> > contiguous physical memory. Or were you just trying to explain the other side of
> > the problem, and I'm just overthinking it?
> 
> The latter :)

Ok, sorry for the noise then, and thank you for educating me.

Alex