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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D47D1C433EF for ; Wed, 8 Dec 2021 19:13:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233072AbhLHTR1 (ORCPT ); Wed, 8 Dec 2021 14:17:27 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52446 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231775AbhLHTR1 (ORCPT ); Wed, 8 Dec 2021 14:17:27 -0500 Received: from mail-yb1-xb31.google.com (mail-yb1-xb31.google.com [IPv6:2607:f8b0:4864:20::b31]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E6A89C061746 for ; Wed, 8 Dec 2021 11:13:54 -0800 (PST) Received: by mail-yb1-xb31.google.com with SMTP id j2so8248202ybg.9 for ; Wed, 08 Dec 2021 11:13:54 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=QHpSZdPvGjFibcYo2hyq0qpLgoBAK/0GYMl9J5pAYXk=; b=SI3J2smLo5pi4P8PXJvn0BlCp8/f1/DlXC7ifXSb43vaejCxj1MNSGEqOywfkZifn+ ACjfyzVDZsJxyw+Q9JVTcnnX3pH99E7bVoyKR/xj5vCS+XuSvky/phPnkeUNEoAbGaxJ +usmBE/+Ift3SFtWlUFNP5iaEpxVCc625ck6WAV85hbUx/UrRCcfIO4ilPq59Ol99n2M oQfMx0130AWkeiqH0PgJUKzdQW1voVkOVTnvM+0mXnrI7DIMPHCYvdHCA7YCZGibJH4m MNeXDITQbGY4FRXglGmrIGCiJCeNJYX6RuyoDBJdZV1YKvjtMYzB21413xa+rz6JPFI/ o1vg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=QHpSZdPvGjFibcYo2hyq0qpLgoBAK/0GYMl9J5pAYXk=; b=j7BEWLBvSXFt69EgUthAmVVa/l9A1TAbXrMABT39weO4KFee9OJh0Mv7ybV8NkNTE+ x6JHWjxtEtKJVI7Szwivs9gVsNjhFcjtflwVeZk2SGjib18RMcxSdcBq2K6T2HMBMAyL uiVgK4WHyjmONGZglcfwvJNxqQa0BoQIpR+Q6uuJI5i8RslsLpiPheld6GCfqva1qIBx z2637bMQAGtekJtesPjeWqmjn2P4bwwqhW0DiCa6qXGGxRaB3WoqzTW0XfsjUzQ3u6oD NP7mzBpUJ8kLUqWD9srlsZjOAc9xbRjKoIxf5Bxh2q74sP4IjeyPcoTeQX0zl09oBLMi 4KEQ== X-Gm-Message-State: AOAM531wFFnwRAlfmhdfyx3lXUxqYsv0k2YQ80PvukyS9+laDdKbQf2T z2xMgMEiqjzvAeVf4bIqCHZw4eafhCdWqLYamnXHpA== X-Google-Smtp-Source: ABdhPJy4xEyJ5m2fUmRBORoeE9LrHENymGsIUrRlGBwpVlG/kqo3yXMLOarkqOZjH4gsYIaW6KAky53598Wj6bobri8= X-Received: by 2002:a25:60d5:: with SMTP id u204mr755449ybb.243.1638990833700; Wed, 08 Dec 2021 11:13:53 -0800 (PST) MIME-Version: 1.0 References: <20211207215031.2251719-1-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Wed, 8 Dec 2021 11:13:42 -0800 Message-ID: Subject: Re: [PATCH v3 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap To: Matthew Wilcox Cc: Michal Hocko , akpm@linux-foundation.org, rientjes@google.com, hannes@cmpxchg.org, guro@fb.com, riel@surriel.com, minchan@kernel.org, kirill@shutemov.name, aarcange@redhat.com, christian@brauner.io, hch@infradead.org, oleg@redhat.com, david@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-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 8, 2021 at 8:50 AM Suren Baghdasaryan wrote: > > On Wed, Dec 8, 2021 at 8:05 AM Matthew Wilcox wrote: > > > > On Wed, Dec 08, 2021 at 04:51:58PM +0100, Michal Hocko wrote: > > > On Wed 08-12-21 15:01:24, Matthew Wilcox wrote: > > > > On Tue, Dec 07, 2021 at 03:08:19PM -0800, Suren Baghdasaryan wrote: > > > > > > > /** > > > > > > > * @close: Called when the VMA is being removed from the MM. > > > > > > > * Context: Caller holds mmap_lock. > > > > > > > > > > BTW, is the caller always required to hold mmap_lock for write or it > > > > > *might* hold it? > > > > > > > > __do_munmap() might hold it for read, thanks to: > > > > > > > > if (downgrade) > > > > mmap_write_downgrade(mm); > > > > > > > > Should probably say: > > > > > > > > * Context: User context. May sleep. Caller holds mmap_lock. > > > > > > > > I don't think we should burden the implementor of the vm_ops with the > > > > knowledge that the VM chooses to not hold the mmap_lock under certain > > > > circumstances when it doesn't matter whether it's holding the mmap_lock > > > > or not. > > > > > > If we document it like that some code might depend on that lock to be > > > held. I think we only want to document that the holder itself is not > > > allowed to take mmap sem or a depending lock. > > > > The only place where we're not currently holding the mmap_lock is at > > task exit, where the mmap_lock is effectively held because nobody else > > can modify the task's mm. Besides, Suren is changing that in this patch > > series anyway, so it will be always true. > > Ok, I'll make it a separate patch after the patch that changes > exit_mmap and this statement will become always true. Sounds > reasonable? Actually, while today vma_ops->close is called with mmap_lock held, I believe we want this comment to reflect the restrictions on the callback itself, not on the user. IOW, we want to say that the callback should not take mmap_lock while the caller might or might not hold it. If so, I think *might* would make more sense here, like this: * Context: User context. May sleep. Caller might hold mmap_lock. WDYT?