linux-kernel - Re: [PATCH v5 09/16] kexec: enable KHO support for memory preservation

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Message-ID: <20250403142438.GF342109@nvidia.com>
Date: Thu, 3 Apr 2025 11:24:38 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Pratyush Yadav <ptyadav@...zon.de>,
	Changyuan Lyu <changyuanl@...gle.com>, linux-kernel@...r.kernel.org,
	graf@...zon.com, akpm@...ux-foundation.org, luto@...nel.org,
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	devicetree@...r.kernel.org, kexec@...ts.infradead.org,
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Subject: Re: [PATCH v5 09/16] kexec: enable KHO support for memory
 preservation

On Thu, Apr 03, 2025 at 04:58:27PM +0300, Mike Rapoport wrote:
> On Thu, Apr 03, 2025 at 08:42:09AM -0300, Jason Gunthorpe wrote:
> > On Wed, Apr 02, 2025 at 07:16:27PM +0000, Pratyush Yadav wrote:
> > > > +int kho_preserve_phys(phys_addr_t phys, size_t size)
> > > > +{
> > > > +	unsigned long pfn = PHYS_PFN(phys), end_pfn = PHYS_PFN(phys + size);
> > > > +	unsigned int order = ilog2(end_pfn - pfn);
> > > 
> > > This caught my eye when playing around with the code. It does not put
> > > any limit on the order, so it can exceed NR_PAGE_ORDERS. Also, when
> > > initializing the page after KHO, we pass the order directly to
> > > prep_compound_page() without sanity checking it. The next kernel might
> > > not support all the orders the current one supports. Perhaps something
> > > to fix?
> > 
> > IMHO we should delete the phys functions until we get a user of them
> 
> The only user of memory tracker in this series uses kho_preserve_phys()

But it really shouldn't. The reserved memory is a completely different
mechanism than buddy allocator preservation. It doesn't even call
kho_restore_phys() those pages, it just feeds the ranges directly to:

+       reserved_mem_add(*p_start, size, name);

The bitmaps should be understood as preserving memory from the buddy
allocator only.

IMHO it should not call kho_preserve_phys() at all.

Jason