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[79.242.58.121]) by smtp.gmail.com with ESMTPSA id z12sm12229644wml.18.2021.08.03.00.48.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 03 Aug 2021 00:48:09 -0700 (PDT) To: Suren Baghdasaryan , akpm@linux-foundation.org Cc: mhocko@kernel.org, mhocko@suse.com, rientjes@google.com, willy@infradead.org, hannes@cmpxchg.org, guro@fb.com, riel@surriel.com, minchan@kernel.org, christian@brauner.io, hch@infradead.org, oleg@redhat.com, jannh@google.com, shakeelb@google.com, luto@kernel.org, christian.brauner@ubuntu.com, fweimer@redhat.com, jengelh@inai.de, timmurray@google.com, linux-api@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com References: <20210802221431.2251210-1-surenb@google.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v4 1/2] mm: introduce process_mrelease system call Message-ID: <95eff329-a7b1-dc2d-026c-fd61e476c846@redhat.com> Date: Tue, 3 Aug 2021 09:48:06 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210802221431.2251210-1-surenb@google.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org [...] > Previously I proposed a number of alternatives to accomplish this: > - https://lore.kernel.org/patchwork/patch/1060407 extending I have no idea how stable these links are. Referencing via message id is the common practice. For this link, we'd use https://lkml.kernel.org/r/20190411014353.113252-3-surenb@google.com/ instead. > 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. I'd condense this description and only reference previous discussions to put a main focus on what this patch actually does. Like " After previous discussions [1, 2, 3] the decision was made to introduce a dedicated system call to cover this use case. ... [1] https://lkml.kernel.org/r/20190411014353.113252-3-surenb@google.com/ " > > The API is as follows, > > int process_mrelease(int pidfd, unsigned int flags); > > DESCRIPTION > The process_mrelease() system call is used to free the memory of > a process which was sent a SIGKILL signal. > > 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_mrelease() returns 0. On error, -1 is > returned and errno is set to indicate the error. > > ERRORS > EBADF pidfd is not a valid PID file descriptor. > > EAGAIN Failed to release part of the address space. > > EINTR The call was interrupted by a signal; see signal(7). > > EINVAL flags is not 0. > > EINVAL The task does not have a pending SIGKILL or its memory is > shared with another process with no pending SIGKILL. Hm, I do wonder if it would make sense to have a mode (e.g., via a flag) to reap all but shared memory from a dying process. Future work. > > ENOSYS This system call is not supported by kernels built with no > MMU support (CONFIG_MMU=n). Maybe "This system call is not supported, for example, without MMU support built into Linux." > > ESRCH The target process does not exist (i.e., it has terminated > and been waited on). > > Signed-off-by: Suren Baghdasaryan > --- > changes in v4: > - Replaced mmap_read_lock() with mmap_read_lock_killable(), per Michal Hocko > - Added EINTR error in the manual pages documentation > > mm/oom_kill.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 58 insertions(+) > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > index c729a4c4a1ac..86727794b0a8 100644 > --- a/mm/oom_kill.c > +++ b/mm/oom_kill.c > @@ -28,6 +28,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -1141,3 +1142,60 @@ void pagefault_out_of_memory(void) > out_of_memory(&oc); > mutex_unlock(&oom_lock); > } > + > +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) > +{ > +#ifdef CONFIG_MMU > + struct mm_struct *mm = NULL; > + struct task_struct *task; > + unsigned int f_flags; > + struct pid *pid; > + long ret = 0; > + > + if (flags != 0) if (flags) > + 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; > + } > + > + /* > + * If the task is dying and in the process of releasing its memory > + * then get its mm. > + */ > + task_lock(task); > + if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) { > + mm = task->mm; > + mmget(mm); > + } > + task_unlock(task); > + if (!mm) { > + ret = -EINVAL; > + goto put_task; > + } > + > + if (mmap_read_lock_killable(mm)) { > + ret = -EINTR; > + goto put_mm; > + } > + if (!__oom_reap_task_mm(mm)) > + ret = -EAGAIN; I'm not an expert on __oom_reap_task_mm(), but the whole approach makes sense to. So feel free to add my Acked-by: David Hildenbrand -- Thanks, David / dhildenb