Re: [RFC 5/7] mm: introduce external memory hinting API

[Michal Hocko <mhocko@kernel.org>]

[Cc linux-api]

On Mon 20-05-19 12:52:52, Minchan Kim wrote:
> There is some usecase that centralized userspace daemon want to give
> a memory hint like MADV_[COOL|COLD] to other process. Android's
> ActivityManagerService is one of them.
> 
> It's similar in spirit to madvise(MADV_WONTNEED), but 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.

Could you expand some more about how this all works? How does the
centralized daemon track respective ranges? How does it synchronize
against parallel modification of the address space etc.

> To solve the issue, this patch introduces new syscall process_madvise(2)
> which works based on pidfd so it could give a hint to the exeternal
> process.
> 
> int process_madvise(int pidfd, void *addr, size_t length, int advise);

OK, this makes some sense from the API point of view. When we have
discussed that at LSFMM I was contemplating about something like that
except the fd would be a VMA fd rather than the process. We could extend
and reuse /proc/<pid>/map_files interface which doesn't support the
anonymous memory right now. 

I am not saying this would be a better interface but I wanted to mention
it here for a further discussion. One slight advantage would be that
you know the exact object that you are operating on because you have a
fd for the VMA and we would have a more straightforward way to reject
operation if the underlying object has changed (e.g. unmapped and reused
for a different mapping).

> All advises madvise provides can be supported in process_madvise, too.
> 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 ptrrace the process could use it successfully.

proc_mem_open model we use for accessing address space via proc sounds
like a good mode. You are doing something similar.

> Please suggest better idea if you have other idea about the permission.
> 
> * from v1r1
>   * use ptrace capability - surenb, dancol
> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  arch/x86/entry/syscalls/syscall_32.tbl |  1 +
>  arch/x86/entry/syscalls/syscall_64.tbl |  1 +
>  include/linux/proc_fs.h                |  1 +
>  include/linux/syscalls.h               |  2 ++
>  include/uapi/asm-generic/unistd.h      |  2 ++
>  kernel/signal.c                        |  2 +-
>  kernel/sys_ni.c                        |  1 +
>  mm/madvise.c                           | 45 ++++++++++++++++++++++++++
>  8 files changed, 54 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 4cd5f982b1e5..5b9dd55d6b57 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -438,3 +438,4 @@
>  425	i386	io_uring_setup		sys_io_uring_setup		__ia32_sys_io_uring_setup
>  426	i386	io_uring_enter		sys_io_uring_enter		__ia32_sys_io_uring_enter
>  427	i386	io_uring_register	sys_io_uring_register		__ia32_sys_io_uring_register
> +428	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 64ca0d06259a..0e5ee78161c9 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -355,6 +355,7 @@
>  425	common	io_uring_setup		__x64_sys_io_uring_setup
>  426	common	io_uring_enter		__x64_sys_io_uring_enter
>  427	common	io_uring_register	__x64_sys_io_uring_register
> +428	common	process_madvise		__x64_sys_process_madvise
>  
>  #
>  # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
> index 52a283ba0465..f8545d7c5218 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -122,6 +122,7 @@ static inline struct pid *tgid_pidfd_to_pid(const struct file *file)
>  
>  #endif /* CONFIG_PROC_FS */
>  
> +extern struct pid *pidfd_to_pid(const struct file *file);
>  struct net;
>  
>  static inline struct proc_dir_entry *proc_net_mkdir(
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index e2870fe1be5b..21c6c9a62006 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -872,6 +872,8 @@ 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 pid_fd, unsigned long start,
> +				size_t len, int behavior);
>  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 dee7292e1df6..7ee82ce04620 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -832,6 +832,8 @@ __SYSCALL(__NR_io_uring_setup, sys_io_uring_setup)
>  __SYSCALL(__NR_io_uring_enter, sys_io_uring_enter)
>  #define __NR_io_uring_register 427
>  __SYSCALL(__NR_io_uring_register, sys_io_uring_register)
> +#define __NR_process_madvise 428
> +__SYSCALL(__NR_process_madvise, sys_process_madvise)
>  
>  #undef __NR_syscalls
>  #define __NR_syscalls 428
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 1c86b78a7597..04e75daab1f8 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -3620,7 +3620,7 @@ static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info)
>  	return copy_siginfo_from_user(kinfo, info);
>  }
>  
> -static struct pid *pidfd_to_pid(const struct file *file)
> +struct pid *pidfd_to_pid(const struct file *file)
>  {
>  	if (file->f_op == &pidfd_fops)
>  		return file->private_data;
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 4d9ae5ea6caf..5277421795ab 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -278,6 +278,7 @@ 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 119e82e1f065..af02aa17e5c1 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -9,6 +9,7 @@
>  #include <linux/mman.h>
>  #include <linux/pagemap.h>
>  #include <linux/page_idle.h>
> +#include <linux/proc_fs.h>
>  #include <linux/syscalls.h>
>  #include <linux/mempolicy.h>
>  #include <linux/page-isolation.h>
> @@ -16,6 +17,7 @@
>  #include <linux/hugetlb.h>
>  #include <linux/falloc.h>
>  #include <linux/sched.h>
> +#include <linux/sched/mm.h>
>  #include <linux/ksm.h>
>  #include <linux/fs.h>
>  #include <linux/file.h>
> @@ -1140,3 +1142,46 @@ SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
>  {
>  	return madvise_core(current, start, len_in, behavior);
>  }
> +
> +SYSCALL_DEFINE4(process_madvise, int, pidfd, unsigned long, start,
> +		size_t, len_in, int, behavior)
> +{
> +	int ret;
> +	struct fd f;
> +	struct pid *pid;
> +	struct task_struct *tsk;
> +	struct mm_struct *mm;
> +
> +	f = fdget(pidfd);
> +	if (!f.file)
> +		return -EBADF;
> +
> +	pid = pidfd_to_pid(f.file);
> +	if (IS_ERR(pid)) {
> +		ret = PTR_ERR(pid);
> +		goto err;
> +	}
> +
> +	ret = -EINVAL;
> +	rcu_read_lock();
> +	tsk = pid_task(pid, PIDTYPE_PID);
> +	if (!tsk) {
> +		rcu_read_unlock();
> +		goto err;
> +	}
> +	get_task_struct(tsk);
> +	rcu_read_unlock();
> +	mm = mm_access(tsk, PTRACE_MODE_ATTACH_REALCREDS);
> +	if (!mm || IS_ERR(mm)) {
> +		ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
> +		if (ret == -EACCES)
> +			ret = -EPERM;
> +		goto err;
> +	}
> +	ret = madvise_core(tsk, start, len_in, behavior);
> +	mmput(mm);
> +	put_task_struct(tsk);
> +err:
> +	fdput(f);
> +	return ret;
> +}
> -- 
> 2.21.0.1020.gf2820cf01a-goog
> 

-- 
Michal Hocko
SUSE Labs