Re: [PATCH] arm64: mmu: no write cache for O_SYNC flag
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2020-03-26 16:55:25
Also in:
lkml
On Thu, Mar 26, 2020 at 09:36:25AM -0700, Li Wang wrote:
reproduce steps:
1.
disable CONFIG_STRICT_DEVMEM in linux kernel
2.
Process A gets a Physical Address of global variable by
"/proc/self/pagemap".
3.
Process B writes a value to the same Physical Address by mmap():
fd=open("/dev/mem",O_SYNC);
Virtual Address=mmap(fd);
problem symptom:
after Process B write a value to the Physical Address,
Process A of the value of global variable does not change.
They both W/R the same Physical Address.
technical reason:
Process B writing the Physical Address is by the Virtual Address,
and the Virtual Address comes from "/dev/mem" and mmap().
In arm64 arch, the Virtual Address has write cache.
So, maybe the value is not written into Physical Address.
fix reason:
giving write cache flag in arm64 is in phys_mem_access_prot():
=====
arch/arm64/mm/mmu.c
phys_mem_access_prot()
{
if (!pfn_valid(pfn))
return pgprot_noncached(vma_prot);
else if (file->f_flags & O_SYNC)
return pgprot_writecombine(vma_prot);
return vma_prot;
}
====
the other arch and the share function drivers/char/mem.c of phys_mem_access_prot()
does not add write cache flag.
So, removing the flag to fix the issueOther architectures may have transparent caches and don't require different attributes.
quoted hunk
Signed-off-by: Li Wang <redacted> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will@kernel.org> Cc: linux-arm-kernel@lists.infradead.org Cc: linux-kernel@vger.kernel.org --- arch/arm64/mm/mmu.c | 2 -- 1 file changed, 2 deletions(-)diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index 128f70852bf3..d7083965ca17 100644 --- a/arch/arm64/mm/mmu.c +++ b/arch/arm64/mm/mmu.c@@ -81,8 +81,6 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn, { if (!pfn_valid(pfn)) return pgprot_noncached(vma_prot); - else if (file->f_flags & O_SYNC) - return pgprot_writecombine(vma_prot); return vma_prot; } EXPORT_SYMBOL(phys_mem_access_prot);
A better solution is for user space not to pass O_SYNC when opening /dev/mem. We've had this ABI for a long time (arch/arm/ and several other architectures do the same), why change it now? -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel