From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-23.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_IN_DEF_DKIM_WL autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 17AE1C432BE for ; Thu, 5 Aug 2021 18:37:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ED4926024A for ; Thu, 5 Aug 2021 18:37:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241365AbhHEShf (ORCPT ); Thu, 5 Aug 2021 14:37:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43326 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232968AbhHEShe (ORCPT ); Thu, 5 Aug 2021 14:37:34 -0400 Received: from mail-yb1-xb2d.google.com (mail-yb1-xb2d.google.com [IPv6:2607:f8b0:4864:20::b2d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 901DBC0613D5 for ; Thu, 5 Aug 2021 11:37:18 -0700 (PDT) Received: by mail-yb1-xb2d.google.com with SMTP id s48so10586098ybi.7 for ; Thu, 05 Aug 2021 11:37:18 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=bNuZ1c9nREEnXSl8bt5cLKPIgLWVSMm+LixG9EWvnFY=; b=fsjk5vdqb3cLg2uKGgjO/Oe/tphZynj/18hayb1IAWVTXxV1DvuaXBFm1EImDNo6zG U2crnR5O94yydOGh6kl6NJNPtEeYyRu91x5tk/hM6WXOXNrGJXWB49+5eIGepgv5RYZb qwgsumpPs9gmmWEBIrOJjQuxoapXVPPkp8QNoIEh4hvdwfEv5rj7+ie3PBlnjOITHpuG gZDXiF9JacbaxGaUfHjFOyPQsBOZxQVJN2wcYzWytbeGqDaBevlbpvsTJIi+s8iI/eqU yYQA1hO44Fdb+OUrnT/TlH3H2vDiUg77roIEFz6bHk0BUp9H9R5Mj3dg9P9fw6ZymLM6 urbA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=bNuZ1c9nREEnXSl8bt5cLKPIgLWVSMm+LixG9EWvnFY=; b=G7wWEN8MgXeE+CRLUNGzB57ytjqdzDvANBTEA09yi0PKC3LUr1D109Oc8as8N7oKYF kzlmlm/Q+kksypQZ7O/TukksxfMlXwUvx5ZvCvsYE5DE5GRdIaqx0T/1NoI6zpmNCYTn K8OIgwEah4czmulV/fjeG8TMOGLepv8lBjuE/2NUONH0z34pQt2BFvh4B2J38jiRglMx mEVi61W/bSAZC+itLYjhiHowH+MWHOgrrKho12lX/pgWVdEPKDo7QNfUpW4z4ZwZQu1T Orybswrn/lG+x68BA4KntWjpPGVIDZ0OqnmNHtHGxBXAT5gNOmyEs3Ty5k64VwcM3Dyd qvdA== X-Gm-Message-State: AOAM530l738xa1ndpFMv4mqIabxCDFFlD3wceoJqlEsOOb/9iK32HD7h qFV9iogP2x6AIEA5MwK6HCxkAeervhEy3YCiQr6LGQ== X-Google-Smtp-Source: ABdhPJzyMfY76OcuYfrzOxc0CqSwpbeqeRxrUs2YZeAuXHjdq5oFOm4vVWVKmrN4l8tpC0P+2z9roxxgQHZhkqtGw1s= X-Received: by 2002:a25:14d6:: with SMTP id 205mr8415464ybu.250.1628188637501; Thu, 05 Aug 2021 11:37:17 -0700 (PDT) MIME-Version: 1.0 References: <20210805170859.2389276-1-surenb@google.com> <46998d10-d0ca-aeeb-8dcd-41b8130fb756@redhat.com> In-Reply-To: From: Suren Baghdasaryan Date: Thu, 5 Aug 2021 11:37:06 -0700 Message-ID: Subject: Re: [PATCH v7 1/2] mm: introduce process_mrelease system call To: Shakeel Butt Cc: David Hildenbrand , Andrew Morton , Michal Hocko , Michal Hocko , David Rientjes , Matthew Wilcox , Johannes Weiner , Roman Gushchin , Rik van Riel , Minchan Kim , Christian Brauner , Christoph Hellwig , Oleg Nesterov , Jann Horn , Andy Lutomirski , Christian Brauner , Florian Weimer , Jan Engelhardt , Tim Murray , Linux API , linux-mm , LKML , kernel-team Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 5, 2021 at 10:56 AM Shakeel Butt wrote: > > On Thu, Aug 5, 2021 at 10:50 AM Suren Baghdasaryan wrote: > > > > On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand wrote: > > > > > > On 05.08.21 19:08, 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_mrelease system call that releases memory of a dying > > > > process from the context of the caller. This way the memory is 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. > > > > > > > > After previous discussions [1, 2, 3] the decision was made [4] to introduce > > > > a dedicated system call to cover this use case. > > > > > > > > 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 > > > > an exiting process. > > > > > > > > The pidfd selects the process referred to by the PID file > > > > descriptor. > > > > (See pidfd_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 memory of the task cannot be released because the > > > > process is not exiting, the address space is shared > > > > with another live process or there is a core dump in > > > > progress. > > > > > > > > ENOSYS 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). > > > > > > > > [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/ > > > > [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/ > > > > [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/ > > > > [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/ > > > > > > > > Signed-off-by: Suren Baghdasaryan > > > > --- > > > > changes in v7: > > > > - Fixed pidfd_open misspelling, per Andrew Morton > > > > - Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko > > > > - Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko > > > > > > > > mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++ > > > > 1 file changed, 73 insertions(+) > > > > > > > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > > > > index c729a4c4a1ac..a4d917b43c73 100644 > > > > --- a/mm/oom_kill.c > > > > +++ b/mm/oom_kill.c > > > > @@ -28,6 +28,7 @@ > > > > #include > > > > #include > > > > #include > > > > +#include > > > > #include > > > > #include > > > > #include > > > > @@ -1141,3 +1142,75 @@ 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; > > > > + struct task_struct *p; > > > > + unsigned int f_flags; > > > > + struct pid *pid; > > > > + long ret = 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. > > > > + */ > > > > + p = find_lock_task_mm(task); > > > > + if (!p) { > > > > + ret = -ESRCH; > > > > + goto put_pid; > > > > + } > > > > + if (task != p) { > > > > + get_task_struct(p); > > > > > > > > > Wouldn't we want to obtain the mm from p ? I thought that was the whole > > > exercise of going via find_lock_task_mm(). > > > > Yes, that's what we do after checking task_will_free_mem(). > > task_will_free_mem() requires us to hold task_lock and > > find_lock_task_mm() achieves that ensuring that mm is still valid, but > > it might return a task other than the original one. That's why we do > > this dance with pinning the new task and unpinning the original one. > > The same dance is performed in __oom_kill_process(). I was > > contemplating adding a parameter to find_lock_task_mm() to request > > this unpin/pin be done within that function but then decided to keep > > it simple for now. > > Did I address your question or did I misunderstand it? > > > > One question I have is why mmget() and not mmgrab()? I see mmgrab() in > oom_kill.c. You are likely right here. The caller's context probably can't be considered a "real user" when reaping the mm. However, we take an mmap_lock shortly after, so not sure if in practice there is much difference. Michal, WDYT? From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-23.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_IN_DEF_DKIM_WL autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9630AC4320A for ; Thu, 5 Aug 2021 18:37:20 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 1F9D660F01 for ; Thu, 5 Aug 2021 18:37:20 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 1F9D660F01 Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kvack.org Received: by kanga.kvack.org (Postfix) id 6CE7B6B006C; Thu, 5 Aug 2021 14:37:19 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 67EA06B0071; Thu, 5 Aug 2021 14:37:19 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 546A78D0001; Thu, 5 Aug 2021 14:37:19 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0138.hostedemail.com [216.40.44.138]) by kanga.kvack.org (Postfix) with ESMTP id 36F796B006C for ; Thu, 5 Aug 2021 14:37:19 -0400 (EDT) Received: from smtpin09.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id D868B182D10A1 for ; Thu, 5 Aug 2021 18:37:18 +0000 (UTC) X-FDA: 78441884556.09.20AA3F4 Received: from mail-yb1-f176.google.com (mail-yb1-f176.google.com [209.85.219.176]) by imf17.hostedemail.com (Postfix) with ESMTP id 8392EF0011F8 for ; Thu, 5 Aug 2021 18:37:18 +0000 (UTC) Received: by mail-yb1-f176.google.com with SMTP id z5so8853515ybj.2 for ; Thu, 05 Aug 2021 11:37:18 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=bNuZ1c9nREEnXSl8bt5cLKPIgLWVSMm+LixG9EWvnFY=; b=fsjk5vdqb3cLg2uKGgjO/Oe/tphZynj/18hayb1IAWVTXxV1DvuaXBFm1EImDNo6zG U2crnR5O94yydOGh6kl6NJNPtEeYyRu91x5tk/hM6WXOXNrGJXWB49+5eIGepgv5RYZb qwgsumpPs9gmmWEBIrOJjQuxoapXVPPkp8QNoIEh4hvdwfEv5rj7+ie3PBlnjOITHpuG gZDXiF9JacbaxGaUfHjFOyPQsBOZxQVJN2wcYzWytbeGqDaBevlbpvsTJIi+s8iI/eqU yYQA1hO44Fdb+OUrnT/TlH3H2vDiUg77roIEFz6bHk0BUp9H9R5Mj3dg9P9fw6ZymLM6 urbA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=bNuZ1c9nREEnXSl8bt5cLKPIgLWVSMm+LixG9EWvnFY=; b=YVUNelN7V9Lnhc8OgA6Z2rQLbUfyidmiEczfoE1rgMUf8BQz9HK/Bykleu8oiWdxQQ tZ3Gayj7n4Fetv/KkNfvCFYOfsACSJTbx9OXUDuW2/IE9ML/EpAqyO4Ir/quUP3dkb++ rz/q5ZMqQ9ZhOJJUgONGjz96l4YNEBLqBrA8SlfYJZc8ItZ1JnvP88qtzqaVXOPc5Jfy 15YTa9nO4BLpwGyYFVxrnTyzqVGzJbHq7WqJklSIzSYl08hXTDjlVfew1rV6Y4T3eo5F BMyXq/S64T2gFfFwN2RdnqISTTlQmS2UHtyBslwAw4zVFOzunDp/4Koz12wbN1Cfc3M5 ovSQ== X-Gm-Message-State: AOAM533tPTVqVVs6/ozBBsBrlva0+QHKHsEopvIIhWlaDdOuQm49cus4 nEPxTck4TumwBaxHZgec4dId4loflGqqVlwXv2tm1A== X-Google-Smtp-Source: ABdhPJzyMfY76OcuYfrzOxc0CqSwpbeqeRxrUs2YZeAuXHjdq5oFOm4vVWVKmrN4l8tpC0P+2z9roxxgQHZhkqtGw1s= X-Received: by 2002:a25:14d6:: with SMTP id 205mr8415464ybu.250.1628188637501; Thu, 05 Aug 2021 11:37:17 -0700 (PDT) MIME-Version: 1.0 References: <20210805170859.2389276-1-surenb@google.com> <46998d10-d0ca-aeeb-8dcd-41b8130fb756@redhat.com> In-Reply-To: From: Suren Baghdasaryan Date: Thu, 5 Aug 2021 11:37:06 -0700 Message-ID: Subject: Re: [PATCH v7 1/2] mm: introduce process_mrelease system call To: Shakeel Butt Cc: David Hildenbrand , Andrew Morton , Michal Hocko , Michal Hocko , David Rientjes , Matthew Wilcox , Johannes Weiner , Roman Gushchin , Rik van Riel , Minchan Kim , Christian Brauner , Christoph Hellwig , Oleg Nesterov , Jann Horn , Andy Lutomirski , Christian Brauner , Florian Weimer , Jan Engelhardt , Tim Murray , Linux API , linux-mm , LKML , kernel-team Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 8392EF0011F8 Authentication-Results: imf17.hostedemail.com; dkim=pass header.d=google.com header.s=20161025 header.b=fsjk5vdq; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf17.hostedemail.com: domain of surenb@google.com designates 209.85.219.176 as permitted sender) smtp.mailfrom=surenb@google.com X-Stat-Signature: ubpubof3kzo44xfos5npt5jpx9tjjn6e X-HE-Tag: 1628188638-509480 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Aug 5, 2021 at 10:56 AM Shakeel Butt wrote: > > On Thu, Aug 5, 2021 at 10:50 AM Suren Baghdasaryan wrote: > > > > On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand wrote: > > > > > > On 05.08.21 19:08, 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_mrelease system call that releases memory of a dying > > > > process from the context of the caller. This way the memory is 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. > > > > > > > > After previous discussions [1, 2, 3] the decision was made [4] to introduce > > > > a dedicated system call to cover this use case. > > > > > > > > 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 > > > > an exiting process. > > > > > > > > The pidfd selects the process referred to by the PID file > > > > descriptor. > > > > (See pidfd_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 memory of the task cannot be released because the > > > > process is not exiting, the address space is shared > > > > with another live process or there is a core dump in > > > > progress. > > > > > > > > ENOSYS 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). > > > > > > > > [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/ > > > > [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/ > > > > [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/ > > > > [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/ > > > > > > > > Signed-off-by: Suren Baghdasaryan > > > > --- > > > > changes in v7: > > > > - Fixed pidfd_open misspelling, per Andrew Morton > > > > - Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko > > > > - Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko > > > > > > > > mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++ > > > > 1 file changed, 73 insertions(+) > > > > > > > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > > > > index c729a4c4a1ac..a4d917b43c73 100644 > > > > --- a/mm/oom_kill.c > > > > +++ b/mm/oom_kill.c > > > > @@ -28,6 +28,7 @@ > > > > #include > > > > #include > > > > #include > > > > +#include > > > > #include > > > > #include > > > > #include > > > > @@ -1141,3 +1142,75 @@ 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; > > > > + struct task_struct *p; > > > > + unsigned int f_flags; > > > > + struct pid *pid; > > > > + long ret = 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. > > > > + */ > > > > + p = find_lock_task_mm(task); > > > > + if (!p) { > > > > + ret = -ESRCH; > > > > + goto put_pid; > > > > + } > > > > + if (task != p) { > > > > + get_task_struct(p); > > > > > > > > > Wouldn't we want to obtain the mm from p ? I thought that was the whole > > > exercise of going via find_lock_task_mm(). > > > > Yes, that's what we do after checking task_will_free_mem(). > > task_will_free_mem() requires us to hold task_lock and > > find_lock_task_mm() achieves that ensuring that mm is still valid, but > > it might return a task other than the original one. That's why we do > > this dance with pinning the new task and unpinning the original one. > > The same dance is performed in __oom_kill_process(). I was > > contemplating adding a parameter to find_lock_task_mm() to request > > this unpin/pin be done within that function but then decided to keep > > it simple for now. > > Did I address your question or did I misunderstand it? > > > > One question I have is why mmget() and not mmgrab()? I see mmgrab() in > oom_kill.c. You are likely right here. The caller's context probably can't be considered a "real user" when reaping the mm. However, we take an mmap_lock shortly after, so not sure if in practice there is much difference. Michal, WDYT?