On 14 April 2016 at 17:25, Will Deacon <will.deacon@xxxxxxx> wrote: > On Wed, Mar 30, 2016 at 04:46:04PM +0200, Ard Biesheuvel wrote: >> The open coded conversion from struct page address to virtual address in >> lowmem_page_address() involves an intermediate conversion step to pfn >> number/physical address. Since the placement of the struct page array >> relative to the linear mapping may be completely independent from the >> placement of physical RAM (as is that case for arm64 after commit >> dfd55ad85e 'arm64: vmemmap: use virtual projection of linear region'), >> the conversion to physical address and back again should factor out of >> the equation, but unfortunately, the shifting and pointer arithmetic >> involved prevent this from happening, and the resulting calculation >> essentially subtracts the address of the start of physical memory and >> adds it back again, in a way that prevents the compiler from optimizing >> it away. >> >> Since the start of physical memory is not a build time constant on arm64, >> the resulting conversion involves an unnecessary memory access, which >> we would like to get rid of. So replace the open coded conversion with >> a call to page_to_virt(), and use the open coded conversion as its >> default definition, to be overriden by the architecture, if desired. >> The existing arch specific definitions of page_to_virt are all equivalent >> to this default definition, so by itself this patch is a no-op. >> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> > > Acked-by: Will Deacon <will.deacon@xxxxxxx> > > I assume you'll post this patch (and the nios2/openrisc) patches as > individual patches targetting the relevant trees? > Sure, as they are completely independent from the rest of the series. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>