linux-kernel - [PATCH] arm64/mm: don't WARN when alloc/free-ing device private pages

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Message-ID: <20230406040515.383238-1-jhubbard@nvidia.com>
Date:   Wed, 5 Apr 2023 21:05:15 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Mark Rutland <mark.rutland@....com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Feiyang Chen <chenfeiyang@...ngson.cn>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>, <stable@...r.kernel.org>
Subject: [PATCH] arm64/mm: don't WARN when alloc/free-ing device private pages

Although CONFIG_DEVICE_PRIVATE and hmm_range_fault() and related
functionality was first developed on x86, it also works on arm64.
However, when trying this out on an arm64 system, it turns out that
there is a massive slowdown during the setup and teardown phases.

This slowdown is due to lots of calls to WARN_ON()'s that are checking
for pages that are out of the physical range for the CPU. However,
that's a design feature of device private pages: they are specfically
chosen in order to be outside of the range of the CPU's true physical
pages.

x86 doesn't have this warning. It only checks that pages are properly
aligned. I've shown a comparison below between x86 (which works well)
and arm64 (which has these warnings).

memunmap_pages()
  pageunmap_range()
    if (pgmap->type == MEMORY_DEVICE_PRIVATE)
      __remove_pages()
        __remove_section()
          sparse_remove_section()
            section_deactivate()
              depopulate_section_memmap()
                /* arch/arm64/mm/mmu.c */
                vmemmap_free()
                {
                  WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
                  ...
                }

                /* arch/x86/mm/init_64.c */
                vmemmap_free()
                {
                  VM_BUG_ON(!PAGE_ALIGNED(start));
                  VM_BUG_ON(!PAGE_ALIGNED(end));
                  ...
                }

So, the warning is a false positive for this case. Therefore, skip the
warning if CONFIG_DEVICE_PRIVATE is set.

Signed-off-by: John Hubbard <jhubbard@...dia.com>
cc: <stable@...r.kernel.org>
---
 arch/arm64/mm/mmu.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 6f9d8898a025..d5c9b611a8d1 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1157,8 +1157,10 @@ int __meminit vmemmap_check_pmd(pmd_t *pmdp, int node,
 int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 		struct vmem_altmap *altmap)
 {
+/* Device private pages are outside of the CPU's physical page range. */
+#ifndef CONFIG_DEVICE_PRIVATE
 	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
-
+#endif
 	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
 		return vmemmap_populate_basepages(start, end, node, altmap);
 	else
@@ -1169,8 +1171,10 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 void vmemmap_free(unsigned long start, unsigned long end,
 		struct vmem_altmap *altmap)
 {
+/* Device private pages are outside of the CPU's physical page range. */
+#ifndef CONFIG_DEVICE_PRIVATE
 	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
-
+#endif
 	unmap_hotplug_range(start, end, true, altmap);
 	free_empty_tables(start, end, VMEMMAP_START, VMEMMAP_END);
 }
-- 
2.40.0