Re: [PATCH v2 3/4] arm64: mm: make vmemmap region a projection of the linear region

On Tue, 10 Nov 2020 at 13:55, Geert Uytterhoeven [off-list ref] wrote:

Hi Ard,

On Thu, Oct 8, 2020 at 5:43 PM Ard Biesheuvel [off-list ref] wrote:

quoted

Now that we have reverted the introduction of the vmemmap struct page
pointer and the separate physvirt_offset, we can simplify things further,
and place the vmemmap region in the VA space in such a way that virtual
to page translations and vice versa can be implemented using a single
arithmetic shift.

One happy coincidence resulting from this is that the 48-bit/4k and
52-bit/64k configurations (which are assumed to be the two most
prevalent) end up with the same placement of the vmemmap region. In
a subsequent patch, we will take advantage of this, and unify the
memory maps even more.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

This is now commit 8c96400d6a39be76 ("arm64: mm: make vmemmap region a
projection of the linear region") in arm64/for-next/core.

quoted

--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c

@@ -504,6 +504,8 @@ static void __init free_unused_memmap(void)
  */
 void __init mem_init(void)
 {
+       BUILD_BUG_ON(!is_power_of_2(sizeof(struct page)));

This check is triggering for me.

If CONFIG_MEMCG=n, sizeof(struct page) = 56.

If CONFIG_HAVE_ALIGNED_STRUCT_PAGE=y, this is mitigated by
the explicit alignment:

    #ifdef CONFIG_HAVE_ALIGNED_STRUCT_PAGE
    #define _struct_page_alignment  __aligned(2 * sizeof(unsigned long))
    #else
    #define _struct_page_alignment
    #endif

    struct page { ... } _struct_page_alignment;

However, HAVE_ALIGNED_STRUCT_PAGE is selected only if SLUB,
while my .config is using SLAB.

Thanks for the report. I will look into this.

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