Date:   Tue, 29 Jun 2021 15:13:58 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, mhocko@...nel.org, mhocko@...e.com,
        rientjes@...gle.com, willy@...radead.org, hannes@...xchg.org,
        guro@...com, riel@...riel.com, minchan@...nel.org,
        christian@...uner.io, hch@...radead.org, oleg@...hat.com,
        david@...hat.com, jannh@...gle.com, shakeelb@...gle.com,
        timmurray@...gle.com, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 1/1] mm: introduce process_reap system call

On Wed, Jun 23, 2021 at 12:28:22PM -0700, Suren Baghdasaryan wrote:
> In modern systems it's not unusual to have a system component monitoring
> memory conditions of the system and tasked with keeping system memory
> pressure under control. One way to accomplish that is to kill
> non-essential processes to free up memory for more important ones.
> Examples of this are Facebook's OOM killer daemon called oomd and
> Android's low memory killer daemon called lmkd.
> For such system component it's important to be able to free memory
> quickly and efficiently. Unfortunately the time process takes to free
> up its memory after receiving a SIGKILL might vary based on the state
> of the process (uninterruptible sleep), size and OPP level of the core
> the process is running. A mechanism to free resources of the target
> process in a more predictable way would improve system's ability to
> control its memory pressure.
> Introduce process_reap system call that reclaims memory of a dying process
> from the context of the caller. This way the memory in freed in a more
> controllable way with CPU affinity and priority of the caller. The workload
> of freeing the memory will also be charged to the caller.
> The operation is allowed only on a dying process.
> 
> Previously I proposed a number of alternatives to accomplish this:
> - https://lore.kernel.org/patchwork/patch/1060407 extending
> pidfd_send_signal to allow memory reaping using oom_reaper thread;
> - https://lore.kernel.org/patchwork/patch/1338196 extending
> pidfd_send_signal to reap memory of the target process synchronously from
> the context of the caller;
> - https://lore.kernel.org/patchwork/patch/1344419/ to add MADV_DONTNEED
> support for process_madvise implementing synchronous memory reaping.
> 
> The end of the last discussion culminated with suggestion to introduce a
> dedicated system call (https://lore.kernel.org/patchwork/patch/1344418/#1553875)
> The reasoning was that the new variant of process_madvise
>   a) does not work on an address range
>   b) is destructive
>   c) doesn't share much code at all with the rest of process_madvise
> From the userspace point of view it was awkward and inconvenient to provide
> memory range for this operation that operates on the entire address space.
> Using special flags or address values to specify the entire address space
> was too hacky.
> 
> The API is as follows,
> 
>           int process_reap(int pidfd, unsigned int flags);
> 
>         DESCRIPTION
>           The process_reap() system call is used to free the memory of a
>           dying process.
> 
>           The pidfd selects the process referred to by the PID file
>           descriptor.
>           (See pidofd_open(2) for further information)
> 
>           The flags argument is reserved for future use; currently, this
>           argument must be specified as 0.
> 
>         RETURN VALUE
>           On success, process_reap() returns 0. On error, -1 is returned
>           and errno is set to indicate the error.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  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                    |  1 +
>  include/uapi/asm-generic/unistd.h           |  4 +-
>  kernel/sys_ni.c                             |  1 +
>  mm/oom_kill.c                               | 50 +++++++++++++++++++++
>  22 files changed, 74 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
> index 3000a2e8ee21..14b9e81d2fc4 100644
> --- a/arch/alpha/kernel/syscalls/syscall.tbl
> +++ b/arch/alpha/kernel/syscalls/syscall.tbl
> @@ -486,3 +486,4 @@
>  554	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  555	common	landlock_add_rule		sys_landlock_add_rule
>  556	common	landlock_restrict_self		sys_landlock_restrict_self
> +557	common	process_reap			sys_process_reap
> diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
> index 28e03b5fec00..889b78d0f63f 100644
> --- a/arch/arm/tools/syscall.tbl
> +++ b/arch/arm/tools/syscall.tbl
> @@ -460,3 +460,4 @@
>  444	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	common	landlock_add_rule		sys_landlock_add_rule
>  446	common	landlock_restrict_self		sys_landlock_restrict_self
> +447	common	process_reap			sys_process_reap
> diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
> index 727bfc3be99b..fb7a0be2f3d9 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		447
> +#define __NR_compat_syscalls		448
>  #endif
>  
>  #define __ARCH_WANT_SYS_CLONE
> diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
> index 5dab69d2c22b..80593454173e 100644
> --- a/arch/arm64/include/asm/unistd32.h
> +++ b/arch/arm64/include/asm/unistd32.h
> @@ -900,6 +900,8 @@ __SYSCALL(__NR_landlock_create_ruleset, sys_landlock_create_ruleset)
>  __SYSCALL(__NR_landlock_add_rule, sys_landlock_add_rule)
>  #define __NR_landlock_restrict_self 446
>  __SYSCALL(__NR_landlock_restrict_self, sys_landlock_restrict_self)
> +#define __NR_process_reap 447
> +__SYSCALL(__NR_process_reap, sys_process_reap)
>  
>  /*
>   * 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 bb11fe4c875a..6c94feedf086 100644
> --- a/arch/ia64/kernel/syscalls/syscall.tbl
> +++ b/arch/ia64/kernel/syscalls/syscall.tbl
> @@ -367,3 +367,4 @@
>  444	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	common	landlock_add_rule		sys_landlock_add_rule
>  446	common	landlock_restrict_self		sys_landlock_restrict_self
> +447	common	process_reap			sys_process_reap
> diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
> index 79c2d24c89dd..e80a7fa55696 100644
> --- a/arch/m68k/kernel/syscalls/syscall.tbl
> +++ b/arch/m68k/kernel/syscalls/syscall.tbl
> @@ -446,3 +446,4 @@
>  444	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	common	landlock_add_rule		sys_landlock_add_rule
>  446	common	landlock_restrict_self		sys_landlock_restrict_self
> +447	common	process_reap			sys_process_reap
> diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
> index b11395a20c20..511b2bd61fc1 100644
> --- a/arch/microblaze/kernel/syscalls/syscall.tbl
> +++ b/arch/microblaze/kernel/syscalls/syscall.tbl
> @@ -452,3 +452,4 @@
>  444	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	common	landlock_add_rule		sys_landlock_add_rule
>  446	common	landlock_restrict_self		sys_landlock_restrict_self
> +447	common	process_reap			sys_process_reap
> diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
> index 9220909526f9..1775704c6a24 100644
> --- a/arch/mips/kernel/syscalls/syscall_n32.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
> @@ -385,3 +385,4 @@
>  444	n32	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	n32	landlock_add_rule		sys_landlock_add_rule
>  446	n32	landlock_restrict_self		sys_landlock_restrict_self
> +447	n32	process_reap			sys_process_reap
> diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
> index 9cd1c34f31b5..d769daca3f79 100644
> --- a/arch/mips/kernel/syscalls/syscall_n64.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
> @@ -361,3 +361,4 @@
>  444	n64	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	n64	landlock_add_rule		sys_landlock_add_rule
>  446	n64	landlock_restrict_self		sys_landlock_restrict_self
> +447	n64	process_reap			sys_process_reap
> diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
> index d560c467a8c6..1bd2fc056677 100644
> --- a/arch/mips/kernel/syscalls/syscall_o32.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
> @@ -434,3 +434,4 @@
>  444	o32	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	o32	landlock_add_rule		sys_landlock_add_rule
>  446	o32	landlock_restrict_self		sys_landlock_restrict_self
> +447	o32	process_reap			sys_process_reap
> diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
> index aabc37f8cae3..0012561ca557 100644
> --- a/arch/parisc/kernel/syscalls/syscall.tbl
> +++ b/arch/parisc/kernel/syscalls/syscall.tbl
> @@ -444,3 +444,4 @@
>  444	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	common	landlock_add_rule		sys_landlock_add_rule
>  446	common	landlock_restrict_self		sys_landlock_restrict_self
> +447	common	process_reap			sys_process_reap
> diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
> index 8f052ff4058c..89cbcc732b18 100644
> --- a/arch/powerpc/kernel/syscalls/syscall.tbl
> +++ b/arch/powerpc/kernel/syscalls/syscall.tbl
> @@ -526,3 +526,4 @@
>  444	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	common	landlock_add_rule		sys_landlock_add_rule
>  446	common	landlock_restrict_self		sys_landlock_restrict_self
> +447	common	process_reap			sys_process_reap
> diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
> index 0690263df1dd..7ebd4d809b5e 100644
> --- a/arch/s390/kernel/syscalls/syscall.tbl
> +++ b/arch/s390/kernel/syscalls/syscall.tbl
> @@ -449,3 +449,4 @@
>  444  common	landlock_create_ruleset	sys_landlock_create_ruleset	sys_landlock_create_ruleset
>  445  common	landlock_add_rule	sys_landlock_add_rule		sys_landlock_add_rule
>  446  common	landlock_restrict_self	sys_landlock_restrict_self	sys_landlock_restrict_self
> +447  common	process_reap		sys_process_reap		sys_process_reap
> diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
> index 0b91499ebdcf..178fd47b372e 100644
> --- a/arch/sh/kernel/syscalls/syscall.tbl
> +++ b/arch/sh/kernel/syscalls/syscall.tbl
> @@ -449,3 +449,4 @@
>  444	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	common	landlock_add_rule		sys_landlock_add_rule
>  446	common	landlock_restrict_self		sys_landlock_restrict_self
> +447	common	process_reap			sys_process_reap
> diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
> index e34cc30ef22c..faee121b7ae2 100644
> --- a/arch/sparc/kernel/syscalls/syscall.tbl
> +++ b/arch/sparc/kernel/syscalls/syscall.tbl
> @@ -492,3 +492,4 @@
>  444	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	common	landlock_add_rule		sys_landlock_add_rule
>  446	common	landlock_restrict_self		sys_landlock_restrict_self
> +447	common	process_reap			sys_process_reap
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 4bbc267fb36b..cbe070de9884 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -451,3 +451,4 @@
>  444	i386	landlock_create_ruleset	sys_landlock_create_ruleset
>  445	i386	landlock_add_rule	sys_landlock_add_rule
>  446	i386	landlock_restrict_self	sys_landlock_restrict_self
> +447	i386	process_reap		sys_process_reap
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index ce18119ea0d0..e6765646731b 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -368,6 +368,7 @@
>  444	common	landlock_create_ruleset	sys_landlock_create_ruleset
>  445	common	landlock_add_rule	sys_landlock_add_rule
>  446	common	landlock_restrict_self	sys_landlock_restrict_self
> +447	common	process_reap		sys_process_reap
>  
>  #
>  # Due to a historical design error, certain syscalls are numbered differently
> diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
> index fd2f30227d96..f0e9dbee1a5b 100644
> --- a/arch/xtensa/kernel/syscalls/syscall.tbl
> +++ b/arch/xtensa/kernel/syscalls/syscall.tbl
> @@ -417,3 +417,4 @@
>  444	common	landlock_create_ruleset		sys_landlock_create_ruleset
>  445	common	landlock_add_rule		sys_landlock_add_rule
>  446	common	landlock_restrict_self		sys_landlock_restrict_self
> +447	common	process_reap			sys_process_reap
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 050511e8f1f8..b6659e09bf0d 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -915,6 +915,7 @@ asmlinkage long sys_mincore(unsigned long start, size_t len,
>  asmlinkage long sys_madvise(unsigned long start, size_t len, int behavior);
>  asmlinkage long sys_process_madvise(int pidfd, const struct iovec __user *vec,
>  			size_t vlen, int behavior, unsigned int flags);
> +asmlinkage long sys_process_reap(int pidfd, unsigned int 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 d2a942086fcb..b3bf57b928af 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -871,9 +871,11 @@ __SYSCALL(__NR_landlock_create_ruleset, sys_landlock_create_ruleset)
>  __SYSCALL(__NR_landlock_add_rule, sys_landlock_add_rule)
>  #define __NR_landlock_restrict_self 446
>  __SYSCALL(__NR_landlock_restrict_self, sys_landlock_restrict_self)
> +#define __NR_process_reap 447
> +__SYSCALL(__NR_process_reap, sys_process_reap)
>  
>  #undef __NR_syscalls
> -#define __NR_syscalls 447
> +#define __NR_syscalls 448
>  
>  /*
>   * 32 bit systems traditionally used different
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 0ea8128468c3..56eb7c9f8356 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -289,6 +289,7 @@ COND_SYSCALL(munlockall);
>  COND_SYSCALL(mincore);
>  COND_SYSCALL(madvise);
>  COND_SYSCALL(process_madvise);
> +COND_SYSCALL(process_reap);
>  COND_SYSCALL(remap_file_pages);
>  COND_SYSCALL(mbind);
>  COND_SYSCALL_COMPAT(mbind);
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index eefd3f5fde46..0f85a0442fa5 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -28,6 +28,7 @@
>  #include <linux/sched/task.h>
>  #include <linux/sched/debug.h>
>  #include <linux/swap.h>
> +#include <linux/syscalls.h>
>  #include <linux/timex.h>
>  #include <linux/jiffies.h>
>  #include <linux/cpuset.h>
> @@ -1141,3 +1142,52 @@ void pagefault_out_of_memory(void)
>  	out_of_memory(&oc);
>  	mutex_unlock(&oom_lock);
>  }
> +
> +SYSCALL_DEFINE2(process_reap, int, pidfd, unsigned int, flags)

Hey Suren,

Wouldn't
- process_memory_reap()
- process_reap_memory()
- process_mreap()
be better names?

> +{
> +	struct pid *pid;
> +	struct task_struct *task;
> +	struct mm_struct *mm = NULL;
> +	unsigned int f_flags;
> +	long ret = 0;
> +
> +	if (flags != 0)
> +		return -EINVAL;
> +
> +	pid = pidfd_get_pid(pidfd, &f_flags);
> +	if (IS_ERR(pid))
> +		return PTR_ERR(pid);
> +
> +	task = get_pid_task(pid, PIDTYPE_PID);
> +	if (!task) {
> +		ret = -ESRCH;
> +		goto put_pid;
> +	}

You have a similar pattern in process_madvise():

	pid = pidfd_get_pid(pidfd, &f_flags);
	if (IS_ERR(pid)) {
		ret = PTR_ERR(pid);
		goto free_iov;
	}

	task = get_pid_task(pid, PIDTYPE_PID);
	if (!task) {
		ret = -ESRCH;
		goto put_pid;
	}

I'd suggest you add a tiny helper to kernel/pid.c and call it in both
places.

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