Messages in this thread Patch in this message | | | From | Li Wang <> | Subject | [PATCH] arm64: mmu: no write cache for O_SYNC flag | Date | Thu, 26 Mar 2020 09:36:25 -0700 |
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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 issue
Signed-off-by: Li Wang <li.wang@windriver.com> 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); -- 2.24.1
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