Apologies for the wall of text. This is a followup to 'restrict virt_to_page to linear region (instead of __pa)' [1], posted on the 24th of this month. This series applies onto current arm64/for-next/core (5be8b70af1ca 'arm64: lse: deal with clobbered IP registers after branch via PLT') with arm64/fixes/core (dfd55ad85e4a 'arm64: vmemmap: use virtual projection of linear region') merged into it. Complete branch can be found here [2]. In current -next, the implementations of virt_to_page and its converse [lowmem_]page_address resolve to virt_to_page 6f0: b6300180 tbz x0, #38, 720 <bar+0x30> 6f4: 90000001 adrp x1, 0 <memstart_addr> 6f8: 92409400 and x0, x0, #0x3fffffffff 6fc: f9400021 ldr x1, [x1] 700: 8b000020 add x0, x1, x0 704: 370001e1 tbnz w1, #0, 740 <bar+0x50> 708: d34cfc00 lsr x0, x0, #12 70c: d2dff7c1 mov x1, #0xffbe00000000 710: f2ffffe1 movk x1, #0xffff, lsl #48 714: 8b001820 add x0, x1, x0, lsl #6 718: d65f03c0 ret 71c: d503201f nop 720: 90000001 adrp x1, 0 <init_pgd> 724: f9400021 ldr x1, [x1] 728: cb010000 sub x0, x0, x1 72c: d2dff7c1 mov x1, #0xffbe00000000 730: d34cfc00 lsr x0, x0, #12 734: f2ffffe1 movk x1, #0xffff, lsl #48 738: 8b001820 add x0, x1, x0, lsl #6 73c: d65f03c0 ret 740: d4210000 brk #0x800 page_address: 6c0: 90000002 adrp x2, 0 <memstart_addr> 6c4: d2c00841 mov x1, #0x4200000000 6c8: 8b010000 add x0, x0, x1 6cc: 9346fc01 asr x1, x0, #6 6d0: f9400040 ldr x0, [x2] 6d4: d374cc21 lsl x1, x1, #12 6d8: 37000080 tbnz w0, #0, 6e8 <foo+0x28> 6dc: cb000020 sub x0, x1, x0 6e0: b25a6400 orr x0, x0, #0xffffffc000000000 6e4: d65f03c0 ret 6e8: d4210000 brk #0x800 Disappointingly, after cherrypicking commit dfd55ad85e ("arm64: vmemmap: use virtual projection of linear region") from arm64/fixes/core, which should make this translation independent of the physical start of RAM, things don't improve much, and even get slightly worse: virt_to_page 6f8: b6300180 tbz x0, #38, 728 <bar+0x30> 6fc: 90000001 adrp x1, 0 <memstart_addr> 700: 92409400 and x0, x0, #0x3fffffffff 704: f9400021 ldr x1, [x1] 708: 8b000020 add x0, x1, x0 70c: 37000261 tbnz w1, #0, 758 <bar+0x60> 710: d34cfc00 lsr x0, x0, #12 714: d2dff7c2 mov x2, #0xffbe00000000 718: cb813000 sub x0, x0, x1, asr #12 71c: f2ffffe2 movk x2, #0xffff, lsl #48 720: 8b001840 add x0, x2, x0, lsl #6 724: d65f03c0 ret 728: 90000002 adrp x2, 0 <init_pgd> 72c: 90000001 adrp x1, 0 <memstart_addr> 730: f9400042 ldr x2, [x2] 734: f9400021 ldr x1, [x1] 738: cb020000 sub x0, x0, x2 73c: d2dff7c2 mov x2, #0xffbe00000000 740: d34cfc00 lsr x0, x0, #12 744: f2ffffe2 movk x2, #0xffff, lsl #48 748: cb813000 sub x0, x0, x1, asr #12 74c: 8b001840 add x0, x2, x0, lsl #6 750: d65f03c0 ret 754: d503201f nop 758: d4210000 brk #0x800 page_address: 6c0: 90000002 adrp x2, 0 <memstart_addr> 6c4: d2c00841 mov x1, #0x4200000000 6c8: f9400043 ldr x3, [x2] 6cc: 934cfc62 asr x2, x3, #12 6d0: 8b021800 add x0, x0, x2, lsl #6 6d4: 8b010001 add x1, x0, x1 6d8: 9346fc21 asr x1, x1, #6 6dc: d374cc21 lsl x1, x1, #12 6e0: 37000083 tbnz w3, #0, 6f0 <foo+0x30> 6e4: cb030020 sub x0, x1, x3 6e8: b25a6400 orr x0, x0, #0xffffffc000000000 6ec: d65f03c0 ret 6f0: d4210000 brk #0x800 The thing to note here is that the expression is evaluated in a way that does not allow the compiler to eliminate the read of memstart_addr, presumably since it is unaware that its value is aligned to PAGE_SIZE, and that shifting it down and up again by 12 bits produces the exact same value. So let's give the compiler a hand here. First of all, let's reimplement virt_to_page() (patch #6) so that it explicitly translates without taking the physical placement into account. This results in the virt_to_page() translation to only work correctly for addresses above PAGE_OFFSET, but this is a reasonable restriction to impose, even if it means a couple of incorrect uses need to be fixed (patches #1 to #4). If we also, in patch #5, move the vmemmap region right below the linear region (which guarantees that the region is always aligned to a power-of-2 upper bound of its size, which means we can treat VMEMMAP_START as a bitmask rather than an offset), we end up with virt_to_page 6d0: d34c9400 ubfx x0, x0, #12, #26 6d4: d2dff7c1 mov x1, #0xffbe00000000 6d8: f2ffffe1 movk x1, #0xffff, lsl #48 6dc: aa001820 orr x0, x1, x0, lsl #6 6e0: d65f03c0 ret In the same way, we can get page_address to look like this page_address: 6c0: d37a7c00 ubfiz x0, x0, #6, #32 6c4: b25a6400 orr x0, x0, #0xffffffc000000000 6c8: d65f03c0 ret However, in this case, we need to slightly refactor the implementation of lowmem_page_paddress(), since it performs an explicit page-to-pa-to-va translation, rather than going through an opaque arch-defined definition of page_to_virt. (patches #7 to #9) [1] http://thread.gmane.org/gmane.linux.ports.arm.kernel/481327 [2] https://git.linaro.org/people/ard.biesheuvel/linux-arm.git/shortlog/refs/heads/arm64-vmemmap (git url git://git.linaro.org/people/ard.biesheuvel/linux-arm.git arm64-vmemmap) Ard Biesheuvel (9): arm64: vdso: avoid virt_to_page() translations on kernel symbols arm64: mm: free __init memory via the linear mapping arm64: mm: avoid virt_to_page() translation for the zero page arm64: insn: avoid virt_to_page() translations on core kernel symbols arm64: mm: move vmemmap region right below the linear region arm64: mm: restrict virt_to_page() to the linear mapping nios2: use correct void* return type for page_to_virt() openrisc: drop wrongly typed definition of page_to_virt() mm: replace open coded page to virt conversion with page_to_virt() arch/arm64/include/asm/memory.h | 30 ++++++++++++++++++-- arch/arm64/include/asm/pgtable.h | 13 +++------ arch/arm64/kernel/insn.c | 2 +- arch/arm64/kernel/vdso.c | 7 +++-- arch/arm64/mm/dump.c | 16 +++++------ arch/arm64/mm/init.c | 19 +++++++++---- arch/nios2/include/asm/io.h | 1 - arch/nios2/include/asm/page.h | 2 +- arch/nios2/include/asm/pgtable.h | 2 +- arch/openrisc/include/asm/page.h | 2 -- include/linux/mm.h | 6 +++- 11 files changed, 65 insertions(+), 35 deletions(-) -- 2.5.0 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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