Re: [PATCH 2/4] mm: introduce external memory hinting API

From: Kirill Tkhai
Date: Mon Jan 13 2020 - 03:47:36 EST

On 11.01.2020 00:34, Minchan Kim wrote:
> There are usecases that System Management Software(SMS) want to give
> a memory hint to other processes because it's not known to the
> application. In the case of Android, ActivityManagerService daemon
> manges app's life cycle and that daemon must be able to initiate
> reclaim on its own without any app involvement.
>
> To solve the issue, this patch introduces new syscall process_madvise(2).
> It uses pidfd of an external processs to give the hint.
>
> int process_madvise(int pidfd, void *addr, size_t length, int advise,
> unsigned long flag);
>
> 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.
>
> 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 more safe for maintainace rather than
> introducing a buggy syscall but hard to fix it later.
>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
> 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/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 | 5 +-
> kernel/sys_ni.c | 1 +
> mm/madvise.c | 64 +++++++++++++++++++++
> 21 files changed, 89 insertions(+), 2 deletions(-)
>
> diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
> index e56950f23b49..776c61803315 100644
> --- a/arch/alpha/kernel/syscalls/syscall.tbl
> +++ b/arch/alpha/kernel/syscalls/syscall.tbl
> @@ -477,3 +477,4 @@
> # 545 reserved for clone3
> 546 common watch_devices sys_watch_devices
> 547 common openat2 sys_openat2
> +548 common process_madvise sys_process_madvise
> diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
> index 7fb2f4d59210..a43381542276 100644
> --- a/arch/arm/tools/syscall.tbl
> +++ b/arch/arm/tools/syscall.tbl
> @@ -451,3 +451,4 @@
> 435 common clone3 sys_clone3
> 436 common watch_devices sys_watch_devices
> 437 common openat2 sys_openat2
> +438 common process_madvise sys_process_madvise
> diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
> index 8aa00ccb0b96..b722e47377a5 100644
> --- a/arch/arm64/include/asm/unistd.h
> +++ b/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 438
> +#define __NR_compat_syscalls 439
> #endif
>
> #define __ARCH_WANT_SYS_CLONE
> diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
> index 31f0ce25719e..5c82557d408f 100644
> --- a/arch/arm64/include/asm/unistd32.h
> +++ b/arch/arm64/include/asm/unistd32.h
> @@ -883,6 +883,8 @@ __SYSCALL(__NR_clone3, sys_clone3)
> __SYSCALL(__NR_watch_devices, sys_watch_devices)
> #define __NR_openat2 437
> __SYSCALL(__NR_openat2, sys_openat2)
> +#define __NR_openat2 438
> +__SYSCALL(__NR_process_madvise, process_madvise)
>
> /*
> * Please add new compat syscalls above this comment and update
> diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
> index b9aa59931905..c156abc9a298 100644
> --- a/arch/ia64/kernel/syscalls/syscall.tbl
> +++ b/arch/ia64/kernel/syscalls/syscall.tbl
> @@ -358,3 +358,4 @@
> # 435 reserved for clone3
> 436 common watch_devices sys_watch_devices
> 437 common openat2 sys_openat2
> +438 common process_madvise sys_process_madvise
> diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
> index 868c1ef89d35..5b6034b6650f 100644
> --- a/arch/m68k/kernel/syscalls/syscall.tbl
> +++ b/arch/m68k/kernel/syscalls/syscall.tbl
> @@ -437,3 +437,4 @@
> # 435 reserved for clone3
> 436 common watch_devices sys_watch_devices
> 437 common openat2 sys_openat2
> +438 common process_madvise sys_process_madvise
> diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
> index 544b4cef18b3..4bef584af09c 100644
> --- a/arch/microblaze/kernel/syscalls/syscall.tbl
> +++ b/arch/microblaze/kernel/syscalls/syscall.tbl
> @@ -443,3 +443,4 @@
> 435 common clone3 sys_clone3
> 436 common watch_devices sys_watch_devices
> 437 common openat2 sys_openat2
> +438 common process_madvise sys_process_madvise
> diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
> index 05e8aee5dae7..94fbd0fcccce 100644
> --- a/arch/mips/kernel/syscalls/syscall_n32.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
> @@ -376,3 +376,4 @@
> 435 n32 clone3 __sys_clone3
> 436 n32 watch_devices sys_watch_devices
> 437 n32 openat2 sys_openat2
> +437 n32 process_madivse sys_process_madvise

438. And several places below has the same mistake.

> diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
> index 24d6c01328fb..4e6982c429d5 100644
> --- a/arch/mips/kernel/syscalls/syscall_n64.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
> @@ -352,3 +352,4 @@
> 435 n64 clone3 __sys_clone3
> 436 n64 watch_devices sys_watch_devices
> 437 n64 openat2 sys_openat2
> +437 n64 process_madvise sys_process_madvise
> diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
> index 4b5f77a4e1a2..3aa990caf9dc 100644
> --- a/arch/parisc/kernel/syscalls/syscall.tbl
> +++ b/arch/parisc/kernel/syscalls/syscall.tbl
> @@ -435,3 +435,4 @@
> 435 common clone3 sys_clone3_wrapper
> 436 common watch_devices sys_watch_devices
> 437 common openat2 sys_openat2
> +437 common process_madvise sys_process_madvise
> diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
> index 9716dc85a517..30e727a23f33 100644
> --- a/arch/powerpc/kernel/syscalls/syscall.tbl
> +++ b/arch/powerpc/kernel/syscalls/syscall.tbl
> @@ -519,3 +519,4 @@
> 435 nospu clone3 ppc_clone3
> 436 common watch_devices sys_watch_devices
> 437 common openat2 sys_openat2
> +437 common process_madvise sys_process_madvise
> diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
> index 7da330f8b03e..75722e5ff496 100644
> --- a/arch/s390/kernel/syscalls/syscall.tbl
> +++ b/arch/s390/kernel/syscalls/syscall.tbl
> @@ -440,3 +440,4 @@
> 435 common clone3 sys_clone3 sys_clone3
> 436 common watch_devices sys_watch_devices sys_watch_devices
> 437 common openat2 sys_openat2 sys_openat2
> +437 common process_madvise sys_process_madvise sys_process_madvise
> diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
> index bb7e68e25337..7d7bc7befad3 100644
> --- a/arch/sh/kernel/syscalls/syscall.tbl
> +++ b/arch/sh/kernel/syscalls/syscall.tbl
> @@ -440,3 +440,4 @@
> # 435 reserved for clone3
> 436 common watch_devices sys_watch_devices
> 437 common openat2 sys_openat2
> +437 common process_madvise sys_process_madvise
> diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
> index 646a1fad7218..581d331ff62f 100644
> --- a/arch/sparc/kernel/syscalls/syscall.tbl
> +++ b/arch/sparc/kernel/syscalls/syscall.tbl
> @@ -483,3 +483,4 @@
> # 435 reserved for clone3
> 436 common watch_devices sys_watch_devices
> 437 common openat2 sys_openat2
> +437 common process_madvise sys_process_madvise
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 57c53acee290..76a2c266fe7e 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -442,3 +442,4 @@
> 435 i386 clone3 sys_clone3 __ia32_sys_clone3
> 436 i386 watch_devices sys_watch_devices __ia32_sys_watch_devices
> 437 i386 openat2 sys_openat2 __ia32_sys_openat2
> +438 i386 process_madvise sys_process_madvise __ia32_sys_process_madvise
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index 1dd8d21f6500..b697cd8620cb 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -359,6 +359,7 @@
> 435 common clone3 __x64_sys_clone3/ptregs
> 436 common watch_devices __x64_sys_watch_devices
> 437 common openat2 __x64_sys_openat2
> +438 common process_madvise __x64_sys_process_madvise
>
> #
> # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
> index 0f48ab7bd75b..2e9813ecfd7d 100644
> --- a/arch/xtensa/kernel/syscalls/syscall.tbl
> +++ b/arch/xtensa/kernel/syscalls/syscall.tbl
> @@ -408,3 +408,4 @@
> 435 common clone3 sys_clone3
> 436 common watch_devices sys_watch_devices
> 437 common openat2 sys_openat2
> +438 common process_madvise sys_process_madvise
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 433c8c85636e..1b58a11ff49f 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -877,6 +877,8 @@ 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);
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 33f3856a9c3c..4bcd8d366f38 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -856,8 +856,11 @@ __SYSCALL(__NR_watch_devices, sys_watch_devices)
> #define __NR_openat2 437
> __SYSCALL(__NR_openat2, sys_openat2)
>
> +#define __NR_openat2 438
> +__SYSCALL(__NR_process_madvise, sys_process_madvise)
> +
> #undef __NR_syscalls
> -#define __NR_syscalls 438
> +#define __NR_syscalls 439
>
> /*
> * 32 bit systems traditionally used different
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 0e9b275260f8..10ce5eac8b4b 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -281,6 +281,7 @@ 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);
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 0c901de531e4..e15dfb4df7bf 100644
> --- a/mm/madvise.c
> +++ b/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>
> @@ -993,6 +994,18 @@ 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;
> + }
> +}
> +
> /*
> * madvise_common - request behavior hint to address range of the target process
> *
> @@ -1169,3 +1182,54 @@ SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
> {
> return madvise_common(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)

I don't like the interface. The fact we have pidfd does not mean,
we have to use it for new syscalls always. A user may want to set
madvise for specific pid from console and pass pid as argument.
pidfd would be an overkill in this case.
We usually call "kill -9 pid" from console. Why shouldn't process_madvise()
allow this?

I suggent to extend first argument to work with both pid and pidfd.
Look at what we have for waitid(idtype, id_t id, ...) for example:

idtype == P_PID
Wait for the child whose process ID matches id.

idtype == P_PIDFD (since Linux 5.4)
Wait for the child referred to by the PID file descriptor specified in id. (See pidfd_open(2) for further information on
PID file descriptors.)

We may use @flags argument for this.

> +{
> + 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 err;
> + }

>From here:

> + rcu_read_lock();
> + task = pid_task(pid, PIDTYPE_PID);
> + if (!task) {
> + rcu_read_unlock();
> + ret = -ESRCH;
> + goto err;
> + }
> +
> + get_task_struct(task);
> + rcu_read_unlock();

to here your code is a copy of existing get_pid_task(). We should the primitive instead.

> +
> + 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 = madvise_common(task, mm, start, len_in, behavior);
> + mmput(mm);
> +release_task:
> + put_task_struct(task);
> +err:

Maybe s/err:/fdput:/ for uniformity with the above "release_task:"?

> + fdput(f);
> + return ret;
> +}
>