linux-kernel - [RFC PATCH 1/2] arm64: mm: Make virt_addr_valid to check for pfn

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Date:   Wed, 28 Jul 2021 19:44:15 +0300
From:   Oleksandr Tyshchenko <olekstysh@...il.com>
To:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Joey Gouly <joey.gouly@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Juergen Gross <jgross@...e.com>, Julien Grall <julien@....org>,
        Wei Chen <Wei.Chen@....com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: [RFC PATCH 1/2] arm64: mm: Make virt_addr_valid to check for pfn_valid again

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>

The problem is that Arm's implementation of virt_addr_valid()
leads to memblock_is_map_memory() check, which will fail for
ZONE_DEVICE based addresses. But, the pfn_valid() check in turn
is able to cope with ZONE_DEVICE based memory.

You can find a good explanation of that problem at:
https://lore.kernel.org/lkml/1614921898-4099-2-git-send-email-anshuman.khandual@arm.com

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
---
I am not quite sure whether it is a "correct" place and
the change itself, I just partially restored a behaviour before:
https://lore.kernel.org/lkml/20210511100550.28178-4-rppt@kernel.org
So, the target of this patch is to get a feedback how to resolve
this properly if, of course, this really needs to be resolved
(I might miss important bits here).

It is worth mentioning that patch doesn't fix the current code base
(if I am not mistaken, no one calls virt_addr_valid() on Arm64 for
ZONE_DEVICE based addresses at the moment, so it seems that nothing
is broken), the fix is intended for the subsequent patch in this
series that will try to enable Xen's "unpopulated-alloc" usage
on Arm (it was enabled on x86 so far).
Please see:
[RFC PATCH 2/2] xen/unpopulated-alloc: Query hypervisor to provide
unallocated space

The subsequent patch will enable the code where virt_addr_valid()
is used in drivers/xen/unpopulated-alloc.c:fill_list() to check that
a virtual address returned by memremap_pages() is valid.
---
 arch/arm64/include/asm/memory.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 824a365..1a35a44 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -351,7 +351,7 @@ static inline void *phys_to_virt(phys_addr_t x)
 
 #define virt_addr_valid(addr)	({					\
 	__typeof__(addr) __addr = __tag_reset(addr);			\
-	__is_lm_address(__addr) && pfn_is_map_memory(virt_to_pfn(__addr));	\
+	__is_lm_address(__addr) && pfn_valid(virt_to_pfn(__addr));	\
 })
 
 void dump_mem_limit(void);
-- 
2.7.4