From: Minchan Kim <minchan@xxxxxxxxxx> Subject: mm/madvise: introduce process_madvise() syscall: an external memory hinting API There is usecase that System Management Software(SMS) want to give a memory hint like MADV_[COLD|PAGEEOUT] to other processes and in the case of Android, it is the ActivityManagerService. The information required to make the reclaim decision is not known to the app. Instead, it is known to the centralized userspace daemon(ActivityManagerService), and that daemon must be able to initiate reclaim on its own without any app involvement. To solve the issue, this patch introduces a new syscall process_madvise(2). It uses pidfd of an external process to give the hint. int process_madvise(int pidfd, void *addr, size_t length, int advice, unsigned long flags); Since it could affect other process's address range, only privileged process(CAP_SYS_PTRACE) or something else(e.g., being the same UID) gives it the right to ptrace the process could use it successfully. The flag argument is reserved for future use if we need to extend the API. I think supporting all hints madvise has/will supported/support to process_madvise is rather risky. Because we are not sure all hints make sense from external process and implementation for the hint may rely on the caller being in the current context so it could be error-prone. Thus, I just limited hints as MADV_[COLD|PAGEOUT] in this patch. If someone want to add other hints, we could hear hear the usecase and review it for each hint. It's safer for maintenance rather than introducing a buggy syscall but hard to fix it later. Q.1 - Why does any external entity have better knowledge? Quote from Sandeep "For Android, every application (including the special SystemServer) are forked from Zygote. The reason of course is to share as many libraries and classes between the two as possible to benefit from the preloading during boot. After applications start, (almost) all of the APIs end up calling into this SystemServer process over IPC (binder) and back to the application. In a fully running system, the SystemServer monitors every single process periodically to calculate their PSS / RSS and also decides which process is "important" to the user for interactivity. So, because of how these processes start _and_ the fact that the SystemServer is looping to monitor each process, it does tend to *know* which address range of the application is not used / useful. Besides, we can never rely on applications to clean things up themselves. We've had the "hey app1, the system is low on memory, please trim your memory usage down" notifications for a long time[1]. They rely on applications honoring the broadcasts and very few do. So, if we want to avoid the inevitable killing of the application and restarting it, some way to be able to tell the OS about unimportant memory in these applications will be useful. - ssp Q.2 - How to guarantee the race(i.e., object validation) between when giving a hint from an external process and get the hint from the target process? process_madvise operates on the target process's address space as it exists at the instant that process_madvise is called. If the space target process can run between the time the process_madvise process inspects the target process address space and the time that process_madvise is actually called, process_madvise may operate on memory regions that the calling process does not expect. It's the responsibility of the process calling process_madvise to close this race condition. For example, the calling process can suspend the target process with ptrace, SIGSTOP, or the freezer cgroup so that it doesn't have an opportunity to change its own address space before process_madvise is called. Another option is to operate on memory regions that the caller knows a priori will be unchanged in the target process. Yet another option is to accept the race for certain process_madvise calls after reasoning that mistargeting will do no harm. The suggested API itself does not provide synchronization. It also apply other APIs like move_pages, process_vm_write. The race isn't really a problem though. Why is it so wrong to require that callers do their own synchronization in some manner? Nobody objects to write(2) merely because it's possible for two processes to open the same file and clobber each other's writes --- instead, we tell people to use flock or something. Think about mmap. It never guarantees newly allocated address space is still valid when the user tries to access it because other threads could unmap the memory right before. That's where we need synchronization by using other API or design from userside. It shouldn't be part of API itself. If someone needs more fine-grained synchronization rather than process level, there were two ideas suggested - cookie[2] and anon-fd[3]. Both are applicable via using last reserved argument of the API but I don't think it's necessary right now since we have already ways to prevent the race so don't want to add additional complexity with more fine-grained optimization model. To make the API extend, it reserved an unsigned long as last argument so we could support it in future if someone really needs it. Q.3 - Why doesn't ptrace work? Injecting an madvise in the target process using ptrace would not work for us because such injected madvise would have to be executed by the target process, which means that process would have to be runnable and that creates the risk of the abovementioned race and hinting a wrong VMA. Furthermore, we want to act the hint in caller's context, not the callee's, because the callee is usually limited in cpuset/cgroups or even freezed state so they can't act by themselves quick enough, which causes more thrashing/kill. It doesn't work if the target process are ptraced(e.g., strace, debugger, minidump) because a process can have at most one ptracer. [1] https://developer.android.com/topic/performance/memory"; [2] process_getinfo for getting the cookie which is updated whenever vma of process address layout are changed - Daniel Colascione - https://lore.kernel.org/lkml/20190520035254.57579-1-minchan@xxxxxxxxxx/T/#m7694416fd179b2066a2c62b5b139b14e3894e224 [3] anonymous fd which is used for the object(i.e., address range) validation - Michal Hocko - https://lore.kernel.org/lkml/20200120112722.GY18451@xxxxxxxxxxxxxx/ [minchan@xxxxxxxxxx: fix process_madvise build break for arm64] Link: http://lkml.kernel.org/r/20200303145756.GA219683@xxxxxxxxxx [minchan@xxxxxxxxxx: fix build error for mips of process_madvise] Link: http://lkml.kernel.org/r/20200508052517.GA197378@xxxxxxxxxx [akpm@xxxxxxxxxxxxxxxxxxxx: fix patch ordering issue] Link: http://lkml.kernel.org/r/20200302193630.68771-3-minchan@xxxxxxxxxx Link: http://lkml.kernel.org/r/20200508183320.GA125527@xxxxxxxxxx Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx> Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> Cc: Brian Geffon <bgeffon@xxxxxxxxxx> Cc: Christian Brauner <christian@xxxxxxxxxx> Cc: Daniel Colascione <dancol@xxxxxxxxxx> Cc: Jann Horn <jannh@xxxxxxxxxx> Cc: Jens Axboe <axboe@xxxxxxxxx> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: John Dias <joaodias@xxxxxxxxxx> Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Oleksandr Natalenko <oleksandr@xxxxxxxxxx> Cc: Sandeep Patil <sspatil@xxxxxxxxxx> Cc: SeongJae Park <sj38.park@xxxxxxxxx> Cc: SeongJae Park <sjpark@xxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Sonny Rao <sonnyrao@xxxxxxxxxx> Cc: Tim Murray <timmurray@xxxxxxxxxx> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx> Cc: <linux-man@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- arch/alpha/kernel/syscalls/syscall.tbl | 1 arch/arm/tools/syscall.tbl | 1 arch/arm64/include/asm/unistd.h | 2 arch/arm64/include/asm/unistd32.h | 2 arch/ia64/kernel/syscalls/syscall.tbl | 1 arch/m68k/kernel/syscalls/syscall.tbl | 1 arch/microblaze/kernel/syscalls/syscall.tbl | 1 arch/mips/kernel/syscalls/syscall_n32.tbl | 1 arch/mips/kernel/syscalls/syscall_n64.tbl | 1 arch/mips/kernel/syscalls/syscall_o32.tbl | 1 arch/parisc/kernel/syscalls/syscall.tbl | 1 arch/powerpc/kernel/syscalls/syscall.tbl | 1 arch/s390/kernel/syscalls/syscall.tbl | 1 arch/sh/kernel/syscalls/syscall.tbl | 1 arch/sparc/kernel/syscalls/syscall.tbl | 1 arch/x86/entry/syscalls/syscall_32.tbl | 1 arch/x86/entry/syscalls/syscall_64.tbl | 1 arch/xtensa/kernel/syscalls/syscall.tbl | 1 include/linux/syscalls.h | 2 include/uapi/asm-generic/unistd.h | 4 - kernel/sys_ni.c | 1 mm/madvise.c | 64 ++++++++++++++++++ 22 files changed, 89 insertions(+), 2 deletions(-) --- a/arch/alpha/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/alpha/kernel/syscalls/syscall.tbl @@ -478,3 +478,4 @@ 547 common openat2 sys_openat2 548 common pidfd_getfd sys_pidfd_getfd 549 common faccessat2 sys_faccessat2 +550 common process_madvise sys_process_madvise --- a/arch/arm64/include/asm/unistd32.h~mm-introduce-external-memory-hinting-api +++ a/arch/arm64/include/asm/unistd32.h @@ -885,6 +885,8 @@ __SYSCALL(__NR_openat2, sys_openat2) __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd) #define __NR_faccessat2 439 __SYSCALL(__NR_faccessat2, sys_faccessat2) +#define __NR_process_madvise 440 +__SYSCALL(__NR_process_madvise, sys_process_madvise) /* * Please add new compat syscalls above this comment and update --- a/arch/arm64/include/asm/unistd.h~mm-introduce-external-memory-hinting-api +++ a/arch/arm64/include/asm/unistd.h @@ -38,7 +38,7 @@ #define __ARM_NR_compat_set_tls (__ARM_NR_COMPAT_BASE + 5) #define __ARM_NR_COMPAT_END (__ARM_NR_COMPAT_BASE + 0x800) -#define __NR_compat_syscalls 440 +#define __NR_compat_syscalls 441 #endif #define __ARCH_WANT_SYS_CLONE --- a/arch/arm/tools/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/arm/tools/syscall.tbl @@ -452,3 +452,4 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise --- a/arch/ia64/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/ia64/kernel/syscalls/syscall.tbl @@ -359,3 +359,4 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise --- a/arch/m68k/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/m68k/kernel/syscalls/syscall.tbl @@ -438,3 +438,4 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise --- a/arch/microblaze/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/microblaze/kernel/syscalls/syscall.tbl @@ -444,3 +444,4 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise --- a/arch/mips/kernel/syscalls/syscall_n32.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/mips/kernel/syscalls/syscall_n32.tbl @@ -377,3 +377,4 @@ 437 n32 openat2 sys_openat2 438 n32 pidfd_getfd sys_pidfd_getfd 439 n32 faccessat2 sys_faccessat2 +440 n32 process_madvise sys_process_madvise --- a/arch/mips/kernel/syscalls/syscall_n64.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/mips/kernel/syscalls/syscall_n64.tbl @@ -353,3 +353,4 @@ 437 n64 openat2 sys_openat2 438 n64 pidfd_getfd sys_pidfd_getfd 439 n64 faccessat2 sys_faccessat2 +440 n64 process_madvise sys_process_madvise --- a/arch/mips/kernel/syscalls/syscall_o32.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/mips/kernel/syscalls/syscall_o32.tbl @@ -426,3 +426,4 @@ 437 o32 openat2 sys_openat2 438 o32 pidfd_getfd sys_pidfd_getfd 439 o32 faccessat2 sys_faccessat2 +440 o32 process_madvise sys_process_madvise --- a/arch/parisc/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/parisc/kernel/syscalls/syscall.tbl @@ -436,3 +436,4 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise --- a/arch/powerpc/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/powerpc/kernel/syscalls/syscall.tbl @@ -528,3 +528,4 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise --- a/arch/s390/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/s390/kernel/syscalls/syscall.tbl @@ -441,3 +441,4 @@ 437 common openat2 sys_openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise sys_process_madvise --- a/arch/sh/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/sh/kernel/syscalls/syscall.tbl @@ -441,3 +441,4 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise --- a/arch/sparc/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/sparc/kernel/syscalls/syscall.tbl @@ -484,3 +484,4 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise --- a/arch/x86/entry/syscalls/syscall_32.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/x86/entry/syscalls/syscall_32.tbl @@ -443,3 +443,4 @@ 437 i386 openat2 sys_openat2 438 i386 pidfd_getfd sys_pidfd_getfd 439 i386 faccessat2 sys_faccessat2 +440 i386 process_madvise sys_process_madvise --- a/arch/x86/entry/syscalls/syscall_64.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/x86/entry/syscalls/syscall_64.tbl @@ -360,6 +360,7 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise # # x32-specific system call numbers start at 512 to avoid cache impact --- a/arch/xtensa/kernel/syscalls/syscall.tbl~mm-introduce-external-memory-hinting-api +++ a/arch/xtensa/kernel/syscalls/syscall.tbl @@ -409,3 +409,4 @@ 437 common openat2 sys_openat2 438 common pidfd_getfd sys_pidfd_getfd 439 common faccessat2 sys_faccessat2 +440 common process_madvise sys_process_madvise --- a/include/linux/syscalls.h~mm-introduce-external-memory-hinting-api +++ a/include/linux/syscalls.h @@ -878,6 +878,8 @@ asmlinkage long sys_munlockall(void); asmlinkage long sys_mincore(unsigned long start, size_t len, unsigned char __user * vec); asmlinkage long sys_madvise(unsigned long start, size_t len, int behavior); +asmlinkage long sys_process_madvise(int pidfd, unsigned long start, + size_t len, int behavior, unsigned long flags); asmlinkage long sys_remap_file_pages(unsigned long start, unsigned long size, unsigned long prot, unsigned long pgoff, unsigned long flags); --- a/include/uapi/asm-generic/unistd.h~mm-introduce-external-memory-hinting-api +++ a/include/uapi/asm-generic/unistd.h @@ -857,9 +857,11 @@ __SYSCALL(__NR_openat2, sys_openat2) __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd) #define __NR_faccessat2 439 __SYSCALL(__NR_faccessat2, sys_faccessat2) +#define __NR_process_madvise 440 +__SYSCALL(__NR_process_madvise, sys_process_madvise) #undef __NR_syscalls -#define __NR_syscalls 440 +#define __NR_syscalls 441 /* * 32 bit systems traditionally used different --- a/kernel/sys_ni.c~mm-introduce-external-memory-hinting-api +++ a/kernel/sys_ni.c @@ -280,6 +280,7 @@ COND_SYSCALL(mlockall); COND_SYSCALL(munlockall); COND_SYSCALL(mincore); COND_SYSCALL(madvise); +COND_SYSCALL(process_madvise); COND_SYSCALL(remap_file_pages); COND_SYSCALL(mbind); COND_SYSCALL_COMPAT(mbind); --- a/mm/madvise.c~mm-introduce-external-memory-hinting-api +++ a/mm/madvise.c @@ -17,6 +17,7 @@ #include <linux/falloc.h> #include <linux/fadvise.h> #include <linux/sched.h> +#include <linux/sched/mm.h> #include <linux/ksm.h> #include <linux/fs.h> #include <linux/file.h> @@ -995,6 +996,18 @@ madvise_behavior_valid(int behavior) } } +static bool +process_madvise_behavior_valid(int behavior) +{ + switch (behavior) { + case MADV_COLD: + case MADV_PAGEOUT: + return true; + default: + return false; + } +} + /* * The madvise(2) system call. * @@ -1042,6 +1055,11 @@ madvise_behavior_valid(int behavior) * MADV_DONTDUMP - the application wants to prevent pages in the given range * from being included in its core dump. * MADV_DODUMP - cancel MADV_DONTDUMP: no longer exclude from core dump. + * MADV_COLD - the application is not expected to use this memory soon, + * deactivate pages in this range so that they can be reclaimed + * easily if memory pressure hanppens. + * MADV_PAGEOUT - the application is not expected to use this memory soon, + * page out the pages in this range immediately. * * return values: * zero - success @@ -1176,3 +1194,49 @@ SYSCALL_DEFINE3(madvise, unsigned long, { return do_madvise(current, current->mm, start, len_in, behavior); } + +SYSCALL_DEFINE5(process_madvise, int, pidfd, unsigned long, start, + size_t, len_in, int, behavior, unsigned long, flags) +{ + int ret; + struct fd f; + struct pid *pid; + struct task_struct *task; + struct mm_struct *mm; + + if (flags != 0) + return -EINVAL; + + if (!process_madvise_behavior_valid(behavior)) + return -EINVAL; + + f = fdget(pidfd); + if (!f.file) + return -EBADF; + + pid = pidfd_pid(f.file); + if (IS_ERR(pid)) { + ret = PTR_ERR(pid); + goto fdput; + } + + task = get_pid_task(pid, PIDTYPE_PID); + if (!task) { + ret = -ESRCH; + goto fdput; + } + + mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS); + if (IS_ERR_OR_NULL(mm)) { + ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; + goto release_task; + } + + ret = do_madvise(task, mm, start, len_in, behavior); + mmput(mm); +release_task: + put_task_struct(task); +fdput: + fdput(f); + return ret; +} _
