From 90666b8a3d5545f4295d9c2517ad607ce5d45e52 Mon Sep 17 00:00:00 2001 From: Austin Clements Date: Mon, 1 Jan 2018 17:53:59 -0500 Subject: [PATCH] runtime: move comment about address space sizes to malloc.go Currently there's a detailed comment in lfstack_64bit.go about address space limitations on various architectures. Since that's now relevant to malloc, move it to a more prominent place in the documentation for memLimitBits. Updates #10460. Change-Id: If9708291cf3a288057b8b3ba0ba6a59e3602bbd6 Reviewed-on: https://go-review.googlesource.com/85889 Run-TryBot: Austin Clements TryBot-Result: Gobot Gobot Reviewed-by: Rick Hudson --- src/runtime/lfstack_64bit.go | 25 ++++++------------------- src/runtime/malloc.go | 19 ++++++++++++++++++- 2 files changed, 24 insertions(+), 20 deletions(-) diff --git a/src/runtime/lfstack_64bit.go b/src/runtime/lfstack_64bit.go index cf6c02895f..19d8045203 100644 --- a/src/runtime/lfstack_64bit.go +++ b/src/runtime/lfstack_64bit.go @@ -11,30 +11,17 @@ import "unsafe" const ( // addrBits is the number of bits needed to represent a virtual address. // - // In Linux the user address space for each architecture is limited as - // follows (taken from the processor.h file for the architecture): - // - // Architecture Name Maximum Value (exclusive) - // --------------------------------------------------------------------- - // amd64 TASK_SIZE_MAX 0x007ffffffff000 (47 bit addresses) - // arm64 TASK_SIZE_64 0x01000000000000 (48 bit addresses) - // ppc64{,le} TASK_SIZE_USER64 0x00400000000000 (46 bit addresses) - // mips64{,le} TASK_SIZE64 0x00010000000000 (40 bit addresses) - // s390x TASK_SIZE 1<<64 (64 bit addresses) - // - // These values may increase over time. In particular, ppc64 - // and mips64 support arbitrary 64-bit addresses in hardware, - // but Linux imposes the above limits. amd64 has hardware - // support for 57 bit addresses as of 2017 (56 bits for user - // space), but Linux only uses addresses above 1<<47 for - // mappings that explicitly pass a high hint address. + // See memLimitBits for a table of address space sizes on + // various architectures. 48 bits is enough for all + // architectures except s390x. // // On AMD64, virtual addresses are 48-bit (or 57-bit) numbers sign extended to 64. // We shift the address left 16 to eliminate the sign extended part and make // room in the bottom for the count. // - // On s390x, there's not much we can do, so we just hope that - // the kernel doesn't get to really high addresses. + // On s390x, virtual addresses are 64-bit. There's not much we + // can do about this, so we just hope that the kernel doesn't + // get to really high addresses and panic if it does. addrBits = 48 // In addition to the 16 bits taken from the top, we can take 3 from the diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go index a397382291..f3e738116c 100644 --- a/src/runtime/malloc.go +++ b/src/runtime/malloc.go @@ -160,7 +160,24 @@ const ( // // On 64-bit platforms, we limit this to 48 bits because that // is the maximum supported by Linux across all 64-bit - // architectures, with the exception of s390x. + // architectures, with the exception of s390x. Based on + // processor.h: + // + // Architecture Name Maximum Value (exclusive) + // --------------------------------------------------------------------- + // amd64 TASK_SIZE_MAX 0x007ffffffff000 (47 bit addresses) + // arm64 TASK_SIZE_64 0x01000000000000 (48 bit addresses) + // ppc64{,le} TASK_SIZE_USER64 0x00400000000000 (46 bit addresses) + // mips64{,le} TASK_SIZE64 0x00010000000000 (40 bit addresses) + // s390x TASK_SIZE 1<<64 (64 bit addresses) + // + // These values may increase over time. In particular, ppc64 + // and mips64 support arbitrary 64-bit addresses in hardware, + // but Linux imposes the above limits. amd64 has hardware + // support for 57 bit addresses as of 2017 (56 bits for user + // space), but Linux only uses addresses above 1<<47 for + // mappings that explicitly pass a high hint address. + // // s390x supports full 64-bit addresses, but the allocator // will panic in the unlikely event we exceed 48 bits. //