[PATCH v6 2/7] mm: introduce external memory hinting API

[Minchan Kim <minchan@kernel.org>]

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.

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 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 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.

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 calle because calle 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@kernel.org/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@dhcp22.suse.cz/

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 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           |  4 +-
 kernel/sys_ni.c                             |  1 +
 mm/madvise.c                                | 64 +++++++++++++++++++++
 21 files changed, 88 insertions(+), 2 deletions(-)

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index 36d42da7466a..c82952e6fb80 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -477,3 +477,4 @@
 # 545 reserved for clone3
 547	common	openat2				sys_openat2
 548	common	pidfd_getfd			sys_pidfd_getfd
+549	common	process_madvise			sys_process_madvise
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index 4d1cf74a2caa..54c2719fec46 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -451,3 +451,4 @@
 435	common	clone3				sys_clone3
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
+439	common	process_madvise			sys_process_madvise
diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 1dd22da1c3a9..75f04a1023be 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		439
+#define __NR_compat_syscalls		440
 #endif
 
 #define __ARCH_WANT_SYS_CLONE
diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index c1c61635f89c..2a27be7a1f91 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_openat2, sys_openat2)
 #define __NR_pidfd_getfd 438
 __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
+#define __NR_process_madvise 439
+__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 042911e670b8..9524af1c318c 100644
--- a/arch/ia64/kernel/syscalls/syscall.tbl
+++ b/arch/ia64/kernel/syscalls/syscall.tbl
@@ -358,3 +358,4 @@
 # 435 reserved for clone3
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
+439	common	process_madvise			sys_process_madvise
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index f4f49fcb76d0..8197050c097c 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -437,3 +437,4 @@
 435	common	clone3				__sys_clone3
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
+439	common	process_madvise			sys_process_madvise
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index 4c67b11f9c9e..c5b6c8afe445 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -443,3 +443,4 @@
 435	common	clone3				sys_clone3
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
+439	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 1f9e8ad636cc..8ec8c558aa9c 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
 437	n32	openat2				sys_openat2
 438	n32	pidfd_getfd			sys_pidfd_getfd
+439	n32	process_madvise			sys_process_madvise
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index c0b9d802dbf6..0078f891bb92 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
 437	n64	openat2				sys_openat2
 438	n64	pidfd_getfd			sys_pidfd_getfd
+439	n64	process_madvise			sys_process_madvise
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index 52a15f5cd130..09c3b5dc6855 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
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
+439	common	process_madvise			sys_process_madvise
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index 35b61bfc1b1a..97eac48c2937 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -519,3 +519,4 @@
 435	nospu	clone3				ppc_clone3
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
+439	common	process_madvise			sys_process_madvise
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index bd7bd3581a0f..8dc8bfd958ea 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
 437  common	openat2			sys_openat2			sys_openat2
 438  common	pidfd_getfd		sys_pidfd_getfd			sys_pidfd_getfd
+439  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 c7a30fcd135f..e69d98040777 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -440,3 +440,4 @@
 # 435 reserved for clone3
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
+439	common	process_madvise			sys_process_madvise
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index f13615ecdecc..6f6e66dd51f9 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -483,3 +483,4 @@
 # 435 reserved for clone3
 437	common	openat2			sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
+439	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 c17cb77eb150..1b2184549e27 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
 437	i386	openat2			sys_openat2			__ia32_sys_openat2
 438	i386	pidfd_getfd		sys_pidfd_getfd			__ia32_sys_pidfd_getfd
+439	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 44d510bc9b78..82d60eb1e00d 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
 437	common	openat2			__x64_sys_openat2
 438	common	pidfd_getfd		__x64_sys_pidfd_getfd
+439	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 85a9ab1bc04d..165cae047770 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -408,3 +408,4 @@
 435	common	clone3				sys_clone3
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
+439	common	process_madvise			sys_process_madvise
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 1815065d52f3..e4cd2c2f8bb4 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -876,6 +876,8 @@ 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 3a3201e4618e..85d8c9376a63 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -855,9 +855,11 @@ __SYSCALL(__NR_clone3, sys_clone3)
 __SYSCALL(__NR_openat2, sys_openat2)
 #define __NR_pidfd_getfd 438
 __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
+#define __NR_pidfd_getfd 439
+__SYSCALL(__NR_process_madvise, sys_process_madvise)
 
 #undef __NR_syscalls
-#define __NR_syscalls 439
+#define __NR_syscalls 440
 
 /*
  * 32 bit systems traditionally used different
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 3b69a560a7ac..6c7332776e8e 100644
--- a/kernel/sys_ni.c
+++ b/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);
diff --git a/mm/madvise.c b/mm/madvise.c
index f75c86b6c463..f29155b8185d 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>
@@ -986,6 +987,18 @@ 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.
  *
@@ -1033,6 +1046,11 @@ 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 uses the memory less so the kernel can
+ *		deactivate the memory to evict them quickly when the memory
+ *		pressure happen.
+ *  MADV_PAGEOUT - the application uses the memroy very rarely so kernel can
+ *		page out the memory instantly.
  *
  * return values:
  *  zero    - success
@@ -1150,3 +1168,49 @@ SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
 {
 	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;
+}
-- 
2.25.0.265.gbab2e86ba0-goog