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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 issue Other architectures may have transparent caches and don't require different attributes. > Signed-off-by: Li Wang > Cc: Catalin Marinas > Cc: Will Deacon > 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