From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8EFA026CE1E; Thu, 25 Jun 2026 11:43:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782387796; cv=none; b=feCuAeHlJexpvNAFEdbN/ZYlscdkhQRow9R3bN0PzYrzfXgSL5BhFuKqaT40AnhO3nAz/29LrNez+J4/7sql1dFD/QnAiOuEghEsgTOcik+OwrehPFxlkEgh0lm+OImEEKchUuTdPtsy9RmxoPm7NCiP2FhMFDg+8aRCTkuG5tE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782387796; c=relaxed/simple; bh=gqQdzqjeMqhD0qB3/uSuuiytjRcKw67JrjyW/Q9nGBs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=qaisygng0JmI5MHaiQMArUeY9B+ieDtbf93dVwF7tMhSK1raSsjyxajWeZgaY1/DA4oOnLEM+6DUO5fBJb3EpJbUd0FmYPIU5ID3+JHZZbAPbCkKAJmxJCs76KCCZ7+81t8CGtOp1y34SnBaGcHSmKtxp8SXxQFHFyId0hhxkx8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Kf+52144; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Kf+52144" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B3BBD1F000E9; Thu, 25 Jun 2026 11:43:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782387795; bh=mCW0ny848an4JhbGV5M+vBhMddaxYL0zCBxrntPtqhM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Kf+52144YXxmSkV3VJTo5FiPwLKlB61+gSnUu2Z7QE6c/l8/fQCzrmDuKVV9i5Fgq s5OGA87ZUjhjDC7bEcH5gE/I3eeNatuhLtCGgFWKXKUceu/f/yi8ETT0K7NsE4SXMW J2ArM6yTtHwrx5MrneOrmNqebpb165FfAZz4gjQY8IXLVW0/X8ArNbn2hQGAgixTbX lMFyGbND3NpQZQBZ3MFSPEXxL51e6UEEoJX/oz6fmm4JxN0gI0Evec3ZCp9xM5s+uG SPF6O9RheHzIwSH9VG/sWNzwK32w8FUSNTpJb4TivTSb0yt/ixr+drLbVd6xJ40NT0 CJsG2xyWTwMDw== Message-ID: <9a8e189a-39b4-4571-bdd5-400cf6443204@kernel.org> Date: Thu, 25 Jun 2026 13:43:07 +0200 Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] coredump: Add /proc//coredump_pre_exit for pre-exit before dumping To: Pedro Falcato Cc: Christian Brauner , Mike Rapoport , Lorenzo Stoakes , mjguzik@gmail.com, ebiederm@xmission.com, viro@zeniv.linux.org.uk, jack@suse.cz, jlayton@kernel.org, chuck.lever@oracle.com, alex.aring@gmail.com, arnd@arndb.de, keescook@chromium.org, mcgrof@kernel.org, j.granados@samsung.com, allen.lkml@gmail.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Xin Zhao , linux-mm@kvack.org References: <20260624145552.70143-1-jackzxcui1989@163.com> <20260625-wappnen-drohbrief-wermutstropfen-c53538f01547@brauner> <9105c433-44a7-4e8f-bacb-def93d11a7f2@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/25/26 13:18, Pedro Falcato wrote: > On Thu, Jun 25, 2026 at 12:57:02PM +0200, David Hildenbrand (Arm) wrote: >>> >>> This makes no sense. I think you really need to sit down and think about >>> a design for this that doesn't introduce state machinery for boot, mm, >>> and the VFS in one shot to solve a fringe problem... >> >> Staring at exit_mmap_mapped_shared(), ... this looks rather hacky ("let's fake >> munmap and set some magical flags"). >> >> We're essentially saying "we don't want (pretty much) anything that's MAP_SHARED >> in the coredump". And for some reason someone should configure that, that's a >> rather weird toggle tbh. >> >> And the granularity ("file-backed shared memory") is completely odd. >> >> >> Aren't there other ways we could optimize this internally? >> >> Like, if we know that a process is dead and cannot run anymore, downgrade writes >> to reads (and make sure we block GUP write attempts accordingly), or would that >> also not be sufficient? >> >> >> Another thought: >> >> fs/coredump.c calls get_dump_page(). >> >> get_dump_page() will not fault in any memory. So if a page is not in the page >> tables at the time of the dump, it will not get included in the coredump. Which >> means, that whether most non-anonymous memory will be included in a coredump is >> already like playing the lottery. >> >> This is true for MAP_SHARED file mappings and MAP_PRIVATE file mappings without >> private modifications. >> >> Which makes me wonder: How much is tooling relying on file-backed pages to end >> up in a coredump? > > FWIW this mechanism already exists, see /proc/self/coredump_filter. The > default is bits 0, 1, 4 and 5 (see core(5)), which maps back to no file pages > being dumped to a core dump, apart from ELF headers (these help the debugger > trace back the mapped binary to the debug info using the buildid). > > So the answer to this question is "approximately none" :) > Ah, thanks! vma_dump_size() honors this, and I am sure through some magical routing the information stored in m->dump_size will end up not dumping these pages. Staring at elf_core_dump(), this "unmap some stuff" part is really, really nasty, as it effectively removes the VMAs->segments from the dump. (unless I am missing something important) -- Cheers, David