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 A2B19C433EF for ; Tue, 31 May 2022 21:21:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347959AbiEaVVl (ORCPT ); Tue, 31 May 2022 17:21:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38412 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1347940AbiEaVVk (ORCPT ); Tue, 31 May 2022 17:21:40 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E301A9CF26 for ; Tue, 31 May 2022 14:21:37 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 7029061348 for ; Tue, 31 May 2022 21:21:37 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8CFDAC3411C; Tue, 31 May 2022 21:21:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1654032096; bh=cO1KqRRg7nAGPkmKXfzoG1Wb3A+63KSDV/6mm7sVgAo=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=VsEl+6tGfgURaNr8aWFlkENDrLkYxFNddhF15xufpzfQqoAdExqhnsLbDtHjieXKb 5asKUyQU5H4N0CusZTr2pYi70LfYBl+VxkG93Gft42GD8bNVPrmmNvR2dsS1hZkjg0 1tBTGz4PwDsZZEv0+1MXhJnC3fY2x92Xqpbmygww= Date: Tue, 31 May 2022 14:21:35 -0700 From: Andrew Morton To: Charan Teja Kalla Cc: , , , , , , , Subject: Re: [PATCH RESEND V5,2/2] mm: shmem: implement POSIX_FADV_[WILL|DONT]NEED for shmem Message-Id: <20220531142135.666b1fcf506e4a327af98ff9@linux-foundation.org> In-Reply-To: References: X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 31 Mar 2022 12:08:21 +0530 Charan Teja Kalla wrote: > From: Charan Teja Reddy > > Currently fadvise(2) is supported only for the files that doesn't > associated with noop_backing_dev_info thus for the files, like shmem, > fadvise results into NOP. But then there is file_operations->fadvise() > that lets the file systems to implement their own fadvise > implementation. Use this support to implement some of the POSIX_FADV_XXX > functionality for shmem files. > > This patch aims to implement POSIX_FADV_WILLNEED and POSIX_FADV_DONTNEED > advices to shmem files which can be helpful for the drivers who may want > to manage the shmem pages of the files that are created through > shmem_file_setup[_with_mnt](). An example usecase may be like, driver > can create the shmem file of the size equal to its requirements and > map the pages for DMA and then pass the fd to user. The user who knows > well about the usage of these pages can now decide when these pages are > not required push them to swap through DONTNEED thus free up memory well > in advance rather than relying on the reclaim and use WILLNEED when it > decide that they are useful in the near future. IOW, it lets the clients > to free up/read the memory when it wants to. Is there an actual userspace/driver combination which will use this? Has the new feature been tested in such an arrangement? And if so, which driver(s)? > Another usecase is that GEM > objects which are currently allocated and managed through shmem files > can use vfs_fadvise(DONT|WILLNEED) on shmem fd when the driver comes to > know(like through some hints from user space) that GEM objects are not > going to use/will need in the near future. Again, is this just a theoretical bright idea, or can we be assured that adding this code to the kernel will end up having been useful to our users? > Some questions asked while reviewing this patch: > > Q) Can the same thing be achieved with FD mapped to user and use > madvise? > A) All drivers are not mapping all the shmem fd's to user space and want > to manage them with in the kernel. Ex: shmem memory can be mapped to the > other subsystems and they fill in the data and then give it to other > subsystem for further processing, where, the user mapping is not at all > required. A simple example, memory that is given for gpu subsystem > which can be filled directly and give to display subsystem. And the > respective drivers know well about when to keep that memory in ram or > swap based on may be a user activity. > > Q) Should we add the documentation section in Manual pages? > A) The man[1] pages for the fadvise() whatever says is also applicable > for shmem files. so couldn't feel it correct to add specific to shmem > files separately. > [1] https://linux.die.net/man/2/fadvise > > Q) The proposed semantics of POSIX_FADV_DONTNEED is actually similar to > MADV_PAGEOUT and different from MADV_DONTNEED. This is a user facing API > and this difference will cause confusion? > A) man pages [1] says that "POSIX_FADV_DONTNEED attempts to free cached > pages associated with the specified region." This means on issuing this > FADV, it is expected to free the file cache pages. And it is > implementation defined If the dirty pages may be attempted to writeback. > And the unwritten dirty pages will not be freed. So, FADV_DONTNEED also > covers the semantics of MADV_PAGEOUT for file pages and there is no > purpose of PAGEOUT for file pages.