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

[Christian Brauner <christian@brauner.io>]

On Tue, May 21, 2019 at 08:35:10PM +0900, Minchan Kim wrote:
> On Tue, May 21, 2019 at 11:01:01AM +0200, Christian Brauner wrote:
> > Cc: Jann and Oleg too
> > 
> > On Mon, May 20, 2019 at 12:52:52PM +0900, 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.
> > > 
> > > 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);
> > > 
> > > 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.
> > > 
> > > 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);
> > 
> > pidfd_to_pid() should not be directly exported since this allows
> > /proc/<pid> fds to be used too. That's something we won't be going
> > forward with. All new syscalls should only allow to operate on pidfds
> > created through CLONE_PIDFD or pidfd_open() (cf. [1]).
> 
> Thanks for the information.
> 
> > 
> > So e.g. please export a simple helper like
> > 
> > struct pid *pidfd_to_pid(const struct file *file)
> > {
> >         if (file->f_op == &pidfd_fops)
> >                 return file->private_data;
> > 
> >         return NULL;
> > }
> > 
> > turning the old pidfd_to_pid() into something like:
> > 
> > static struct pid *__fd_to_pid(const struct file *file)
> > {
> >         struct pid *pid;
> > 
> >         pid = pidfd_to_pid(file);
> >         if (pid)
> >                 return pid;
> > 
> >         return tgid_pidfd_to_pid(file);
> > }
> 
> So, I want to clarify what you suggest here.
> 
> 1. modify pidfd_to_pid as what you described above(ie, return NULL
> instead of returning tgid_pidfd_to_pid(file);
> 2. never export pidfd_to_pid
> 3. create wrapper __fd_to_pid which calls pidfd_to_pid internally
> 4. export __fd_to_pid and use it

Sorry, seems I was not clear enough. :)
What I meant was something along the lines of (not compile tested):

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 3c8ef5a199ca..3ccc07010603 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -68,6 +68,10 @@ extern struct pid init_struct_pid;
 
 extern const struct file_operations pidfd_fops;
 
+struct file;
+
+extern struct pid *pidfd_pid(const struct file *file);
+
 static inline struct pid *get_pid(struct pid *pid)
 {
 	if (pid)
diff --git a/kernel/fork.c b/kernel/fork.c
index b4cba953040a..0fcfffaf5fdc 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1711,6 +1711,14 @@ const struct file_operations pidfd_fops = {
 #endif
 };
 
+struct pid *pidfd_pid(const struct file *file)
+{
+	if (file->f_op == &pidfd_fops)
+		return file->private_data;
+
+	return ERR_PTR(-EBADF);
+}
+
 /**
  * pidfd_create() - Create a new pid file descriptor.
  *
diff --git a/kernel/signal.c b/kernel/signal.c
index a1eb44dc9ff5..7fca9d7e1633 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3611,8 +3611,11 @@ static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info)
 
 static struct pid *pidfd_to_pid(const struct file *file)
 {
-	if (file->f_op == &pidfd_fops)
-		return file->private_data;
+	struct pid *pid;
+
+	pid = pidfd_pid(file);
+	if (pid)
+		return pid;
 
 	return tgid_pidfd_to_pid(file);
 }