linux-kernel - Re: [v5 PATCH] arm64: mm: show direct mapping use in /proc/meminfo

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Message-ID: <aXd3ralBsv9hPtfe@willie-the-truck>
Date: Mon, 26 Jan 2026 14:18:21 +0000
From: Will Deacon <will@...nel.org>
To: Yang Shi <yang@...amperecomputing.com>
Cc: catalin.marinas@....com, ryan.roberts@....com, cl@...two.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [v5 PATCH] arm64: mm: show direct mapping use in /proc/meminfo

On Tue, Jan 13, 2026 at 04:36:06PM -0800, Yang Shi wrote:
> On 1/13/26 6:36 AM, Will Deacon wrote:
> > On Tue, Jan 06, 2026 at 04:29:44PM -0800, Yang Shi wrote:
> > > +#if defined(CONFIG_ARM64_4K_PAGES)
> > > +	size[PTE] = "4k";
> > > +	size[CONT_PTE] = "64k";
> > > +	size[PMD] = "2M";
> > > +	size[CONT_PMD] = "32M";
> > > +	size[PUD] = "1G";
> > > +#elif defined(CONFIG_ARM64_16K_PAGES)
> > > +	size[PTE] = "16k";
> > > +	size[CONT_PTE] = "2M";
> > > +	size[PMD] = "32M";
> > > +	size[CONT_PMD] = "1G";
> > > +#elif defined(CONFIG_ARM64_64K_PAGES)
> > > +	size[PTE] = "64k";
> > > +	size[CONT_PTE] = "2M";
> > > +	size[PMD] = "512M";
> > > +	size[CONT_PMD] = "16G";
> > > +#endif
> > > +
> > > +	seq_printf(m, "DirectMap%s:	%8lu kB\n",
> > > +			size[PTE], dm_meminfo[PTE] >> 10);
> > > +	seq_printf(m, "DirectMap%s:	%8lu kB\n",
> > > +			size[CONT_PTE],
> > > +			dm_meminfo[CONT_PTE] >> 10);
> > > +	seq_printf(m, "DirectMap%s:	%8lu kB\n",
> > > +			size[PMD], dm_meminfo[PMD] >> 10);
> > > +	seq_printf(m, "DirectMap%s:	%8lu kB\n",
> > > +			size[CONT_PMD],
> > > +			dm_meminfo[CONT_PMD] >> 10);
> > > +	if (pud_sect_supported())
> > > +		seq_printf(m, "DirectMap%s:	%8lu kB\n",
> > > +			size[PUD], dm_meminfo[PUD] >> 10);
> > This seems a bit brittle to me. If somebody adds support for l1 block
> > mappings for !4k pages in future, they will forget to update this and
> > we'll end up returning kernel stack in /proc/meminfo afaict.
> 
> I can initialize size[PUD] to "NON_SUPPORT" by default. If the case happens,
> /proc/meminfo just shows "DirectMapNON_SUPPORT", then we will notice
> something is missed, but no kernel stack data will be leak.

Or just add the PUD sizes for all the page sizes...

> > > @@ -266,6 +351,17 @@ static int init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
> > >   		    (flags & NO_BLOCK_MAPPINGS) == 0) {
> > >   			pmd_set_huge(pmdp, phys, prot);
> > > +			/*
> > > +			 * It is possible to have mappings allow cont mapping
> > > +			 * but disallow block mapping. For example,
> > > +			 * map_entry_trampoline().
> > > +			 * So we have to increase CONT_PMD and PMD size here
> > > +			 * to avoid double counting.
> > > +			 */
> > > +			if (pgprot_val(prot) & PTE_CONT)
> > > +				dm_meminfo_add(addr, (next - addr), CONT_PMD);
> > > +			else
> > > +				dm_meminfo_add(addr, (next - addr), PMD);
> > I don't understand the comment you're adding here. If somebody passes
> > NO_BLOCK_MAPPINGS then that also prevents contiguous entries except at
> > level 3.
> 
> The comment may be misleading. I meant if we have the accounting code for
> CONT_PMD in alloc_init_cont_pmd(), for example,

I think I'd just drop the comment. The code is clear enough once you
actually read what's going on.

> @@ -433,6 +433,11 @@ static int alloc_init_cont_pmd(pud_t *pudp, unsigned
> long addr,
>                 if (ret)
>                         goto out;
> 
> +               if (pgprot_val(prot) & PTE_CONT)
> +                       dm_meminfo_add(addr, (next - addr), CONT_PMD);
> 
>                 pmdp += pmd_index(next) - pmd_index(addr);
>                 phys += next - addr;
>         } while (addr = next, addr != end);
> 
> If the described case happens, we actually miscount CONT_PMD. So I need to
> check whether it is CONT in init_pmd() instead. If the comment is confusing,
> I can just remove it.
> 
> > It also doesn't look you handle the error case properly when the mapping
> > fails.
> 
> I don't quite get what fail do you mean? pmd_set_huge() doesn't fail. Or you
> meant hotplug fails? If so the hot unplug will decrease the counters, which
> is called in the error handling path.

Sorry, I got confused here and thought that we could end up with a
partially-formed contiguous region but that's not the case. So you can
ignore this comment :)

Will