On Fri, Feb 17, 2017 at 12:02:13PM -0800, Linus Torvalds wrote: > I also get he feeling that the whole thing is unnecessary. I'm > wondering if we should just instead say that the whole 47 vs 56-bit > virtual address is _purely_ about "get_unmapped_area()", and nothing > else. > > IOW, I'm wondering if we can't just say that > > - if the processor and kernel support 56-bit user address space, then > you can *always* use the whole space > > - but by default, get_unmapped_area() will only return mappings that > fit in the 47 bit address space. > > So if you use MAP_FIXED and give an address in the high range, it will > just always work, and the MM will always consider the task size to be > the full address space. > > But for the common case where a process does no use MAP_FIXED, the > kernel will never give a high address by default, and you have to do > the process control thing to say "I want those high addresses". > > Hmm? > > In other words, I'd like to at least start out trying to keep the > differences between the 47-bit and 56-bit models as simple and minimal > as possible. Not make such a big deal out of it. > > We already have "arch_get_unmapped_area()" that controls the whole > "what will non-MAP_FIXED mmap allocations return", so I'd hope that > the above kind of semantics could be done without *any* actual > TASK_SIZE changes _anywhere_ in the VM code. > > Comments? Okay, below is my try on implementing this. I've chosen to respect hint address even without MAP_FIXED, but only if it doesn't collide with other mappings. Otherwise, fallback to look for unmapped area within 47-bit window. Interaction with MPX would requires more work. I'm not yet sure what is the right way to address it. Also Dave noticed that some test-cases from ltp would break with the approach. See for instance hugemmap03. I don't think it matter much as it tests for negative outcome and I don't expect real world application to do anything like this. Test-case that I used to test the patch: #include <stdio.h> #include <sys/mman.h> #define SIZE (2UL << 20) #define LOW_ADDR ((void *) (1UL << 30)) #define HIGH_ADDR ((void *) (1UL << 50)) int main(int argc, char **argv) { void *p; p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); printf("mmap(NULL): %p\n", p); p = mmap(LOW_ADDR, SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); printf("mmap(%p): %p\n", LOW_ADDR, p); p = mmap(HIGH_ADDR, SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); printf("mmap(%p): %p\n", HIGH_ADDR, p); p = mmap(HIGH_ADDR, SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); printf("mmap(%p) again: %p\n", HIGH_ADDR, p); p = mmap(HIGH_ADDR, SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0); printf("mmap(%p, MAP_FIXED): %p\n", HIGH_ADDR, p); return 0; } ------------------------8<--------------------------- diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h index e7f155c3045e..9c6315d9aa34 100644 --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -250,7 +250,7 @@ extern int force_personality32; the loader. We need to make sure that it is out of the way of the program that it will "exec", and that there is sufficient room for the brk. */ -#define ELF_ET_DYN_BASE (TASK_SIZE / 3 * 2) +#define ELF_ET_DYN_BASE (DEFAULT_MAP_WINDOW / 3 * 2) /* This yields a mask that user programs can use to figure out what instruction set this CPU supports. This could be done in user space, diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index e6cfe7ba2d65..492548c87cb1 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -789,6 +789,7 @@ static inline void spin_lock_prefetch(const void *x) */ #define TASK_SIZE PAGE_OFFSET #define TASK_SIZE_MAX TASK_SIZE +#define DEFAULT_MAP_WINDOW TASK_SIZE #define STACK_TOP TASK_SIZE #define STACK_TOP_MAX STACK_TOP @@ -828,7 +829,9 @@ static inline void spin_lock_prefetch(const void *x) * particular problem by preventing anything from being mapped * at the maximum canonical address. */ -#define TASK_SIZE_MAX ((1UL << 47) - PAGE_SIZE) +#define TASK_SIZE_MAX ((1UL << __VIRTUAL_MASK_SHIFT) - PAGE_SIZE) + +#define DEFAULT_MAP_WINDOW ((1UL << 47) - PAGE_SIZE) /* This decides where the kernel will search for a free chunk of vm * space during mmap's. @@ -841,7 +844,7 @@ static inline void spin_lock_prefetch(const void *x) #define TASK_SIZE_OF(child) ((test_tsk_thread_flag(child, TIF_ADDR32)) ? \ IA32_PAGE_OFFSET : TASK_SIZE_MAX) -#define STACK_TOP TASK_SIZE +#define STACK_TOP DEFAULT_MAP_WINDOW #define STACK_TOP_MAX TASK_SIZE_MAX #define INIT_THREAD { \ @@ -863,7 +866,7 @@ extern void start_thread(struct pt_regs *regs, unsigned long new_ip, * This decides where the kernel will search for a free chunk of vm * space during mmap's. */ -#define TASK_UNMAPPED_BASE (PAGE_ALIGN(TASK_SIZE / 3)) +#define TASK_UNMAPPED_BASE (PAGE_ALIGN(DEFAULT_MAP_WINDOW / 3)) #define KSTK_EIP(task) (task_pt_regs(task)->ip) diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c index a55ed63b9f91..7f2e26dca1f2 100644 --- a/arch/x86/kernel/sys_x86_64.c +++ b/arch/x86/kernel/sys_x86_64.c @@ -147,7 +147,7 @@ arch_get_unmapped_area(struct file *filp, unsigned long addr, info.flags = 0; info.length = len; info.low_limit = begin; - info.high_limit = end; + info.high_limit = min(end, DEFAULT_MAP_WINDOW); info.align_mask = 0; info.align_offset = pgoff << PAGE_SHIFT; if (filp) { diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c index 2ae8584b44c7..e1c2ee098be0 100644 --- a/arch/x86/mm/hugetlbpage.c +++ b/arch/x86/mm/hugetlbpage.c @@ -82,7 +82,7 @@ static unsigned long hugetlb_get_unmapped_area_bottomup(struct file *file, info.flags = 0; info.length = len; info.low_limit = current->mm->mmap_legacy_base; - info.high_limit = TASK_SIZE; + info.high_limit = DEFAULT_MAP_WINDOW; info.align_mask = PAGE_MASK & ~huge_page_mask(h); info.align_offset = 0; return vm_unmapped_area(&info); @@ -114,7 +114,7 @@ static unsigned long hugetlb_get_unmapped_area_topdown(struct file *file, VM_BUG_ON(addr != -ENOMEM); info.flags = 0; info.low_limit = TASK_UNMAPPED_BASE; - info.high_limit = TASK_SIZE; + info.high_limit = DEFAULT_MAP_WINDOW; addr = vm_unmapped_area(&info); } diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c index d2dc0438d654..a29a830ad341 100644 --- a/arch/x86/mm/mmap.c +++ b/arch/x86/mm/mmap.c @@ -52,7 +52,7 @@ static unsigned long stack_maxrandom_size(void) * Leave an at least ~128 MB hole with possible stack randomization. */ #define MIN_GAP (128*1024*1024UL + stack_maxrandom_size()) -#define MAX_GAP (TASK_SIZE/6*5) +#define MAX_GAP (DEFAULT_MAP_WINDOW/6*5) static int mmap_is_legacy(void) { @@ -90,7 +90,7 @@ static unsigned long mmap_base(unsigned long rnd) else if (gap > MAX_GAP) gap = MAX_GAP; - return PAGE_ALIGN(TASK_SIZE - gap - rnd); + return PAGE_ALIGN(DEFAULT_MAP_WINDOW - gap - rnd); } /* -- Kirill A. 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