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Date: Thu, 16 Jul 2026 13:52:07 +0300
From: Mike Rapoport <rppt@kernel.org>
To: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: Brendan Jackman <brendan.jackman@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Brendan Jackman <jackmanb@google.com>,
	Lorenzo Stoakes <ljs@kernel.org>,
	"Liam R. Howlett" <liam@infradead.org>,
	Vlastimil Babka <vbabka@kernel.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Michal Hocko <mhocko@suse.com>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/secretmem: disable under HIGHMEM
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In-Reply-To: <5653474e-6692-4353-b5d6-e57b6ab76eaf@kernel.org>

On Thu, Jul 16, 2026 at 12:08:07PM +0200, David Hildenbrand (Arm) wrote:
> On 7/15/26 16:58, Mike Rapoport wrote:
> > On Wed, Jul 15, 2026 at 03:49:39PM +0200, David Hildenbrand (Arm) wrote:
> >> On 7/15/26 15:44, Mike Rapoport wrote:
> >>>
> >>> No, I mean what Brendan's patch did:
> >>>
> >>>  config SECRETMEM
> >>>         default y
> >>>         bool "Enable memfd_secret() system call" if EXPERT
> >>> -       depends on ARCH_HAS_SET_DIRECT_MAP
> >>> +       depends on ARCH_HAS_SET_DIRECT_MAP && !HIGHMEM
> >>>
> >>> We can change gfp_mask as well for the sake of pedancy
> >>
> >> Why not change the gfp mask only then?
> > 
> > Citing myself from a few emails back: 
> > 
> >>> But still with kmap() and friends not being an NOP the promise "kernel does
> >>> not map this memory" does not hold.
> 
> So you're saying that kmap() would perform a mapping for a non-highmem page?
> 
> static inline void *kmap(struct page *page)
> {
> 	void *addr;
> 
> 	might_sleep();
> 	if (!PageHighMem(page))
> 		addr = page_address(page);
> 	else
> 		addr = kmap_high(page);
> 	kmap_flush_tlb((unsigned long)addr);
> 	return addr;
> }
> 
> I'm missing something important.

Yeah, you are right, GFP_USER should do.
 
> -- 
> Cheers,
> 
> David

-- 
Sincerely yours,
Mike.