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 7D135C7EE2F for ; Tue, 13 Jun 2023 03:20:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238840AbjFMDU4 (ORCPT ); Mon, 12 Jun 2023 23:20:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34664 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233437AbjFMDUy (ORCPT ); Mon, 12 Jun 2023 23:20:54 -0400 Received: from out-25.mta0.migadu.com (out-25.mta0.migadu.com [91.218.175.25]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0C0F7A1 for ; Mon, 12 Jun 2023 20:20:51 -0700 (PDT) Message-ID: <28c92418-7f07-978a-0eaa-b0d6329f4133@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1686626449; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=5MQfCFW+KGcTGW7SDJeQhmjQZOiFWG6iCUvwf7MCmkk=; b=L6o0ZfcluUkqGi8xJD+ZC+etlIx/iPSrAF81h0lOFvRuGnFolBWHrxOQtnQpfZkQPT1Fkr TMaoAvkXSeBLbgLbxjvr0L4t3Xwdaj8v3JvzkyXaovqj4fo6DGOekaqZSwJpXPeN8QjlwC IZMtvWLFw/h9gslFGwpQQ9TRmIHNhcE= Date: Tue, 13 Jun 2023 11:20:44 +0800 MIME-Version: 1.0 Subject: Re: [PATCH] fuse: add a new flag to allow shared mmap in FOPEN_DIRECT_IO mode Content-Language: en-US To: Jingbo Xu , Vivek Goyal Cc: fuse-devel@lists.sourceforge.net, miklos@szeredi.hu, bernd.schubert@fastmail.fm, linux-fsdevel@vger.kernel.org References: <20230505081652.43008-1-hao.xu@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Xu In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Hi Jingbo, On 6/13/23 10:56, Jingbo Xu wrote: > > > On 5/6/23 1:01 PM, Hao Xu wrote: >> Hi Vivek, >> >> On 5/6/23 04:37, Vivek Goyal wrote: >>> On Fri, May 05, 2023 at 04:16:52PM +0800, Hao Xu wrote: >>>> From: Hao Xu >>>> >>>> FOPEN_DIRECT_IO is usually set by fuse daemon to indicate need of strong >>>> coherency, e.g. network filesystems. Thus shared mmap is disabled since >>>> it leverages page cache and may write to it, which may cause >>>> inconsistence. But FOPEN_DIRECT_IO can be used not for coherency but to >>>> reduce memory footprint as well, e.g. reduce guest memory usage with >>>> virtiofs. Therefore, add a new flag FOPEN_DIRECT_IO_SHARED_MMAP to allow >>>> shared mmap for these cases. >>>> >>>> Signed-off-by: Hao Xu >>>> --- >>>>   fs/fuse/file.c            | 11 ++++++++--- >>>>   include/uapi/linux/fuse.h |  2 ++ >>>>   2 files changed, 10 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/fs/fuse/file.c b/fs/fuse/file.c >>>> index 89d97f6188e0..655896bdb0d5 100644 >>>> --- a/fs/fuse/file.c >>>> +++ b/fs/fuse/file.c >>>> @@ -161,7 +161,8 @@ struct fuse_file *fuse_file_open(struct >>>> fuse_mount *fm, u64 nodeid, >>>>       } >>>>         if (isdir) >>>> -        ff->open_flags &= ~FOPEN_DIRECT_IO; >>>> +        ff->open_flags &= >>>> +            ~(FOPEN_DIRECT_IO | FOPEN_DIRECT_IO_SHARED_MMAP); >>>>         ff->nodeid = nodeid; >>>>   @@ -2509,8 +2510,12 @@ static int fuse_file_mmap(struct file *file, >>>> struct vm_area_struct *vma) >>>>           return fuse_dax_mmap(file, vma); >>>>         if (ff->open_flags & FOPEN_DIRECT_IO) { >>>> -        /* Can't provide the coherency needed for MAP_SHARED */ >>>> -        if (vma->vm_flags & VM_MAYSHARE) >>>> +        /* Can't provide the coherency needed for MAP_SHARED. >>>> +         * So disable it if FOPEN_DIRECT_IO_SHARED_MMAP is not >>>> +         * set, which means we do need strong coherency. >>>> +         */ >>>> +        if (!(ff->open_flags & FOPEN_DIRECT_IO_SHARED_MMAP) && >>>> +            vma->vm_flags & VM_MAYSHARE) >>>>               return -ENODEV; >>> >>> Can you give an example how this is useful and how do you plan to >>> use it? >>> >>> If goal is not using guest cache (either for saving memory or for cache >>> coherency with other clients) and hence you used FOPEN_DIRECT_IO, >>> then by allowing page cache for mmap(), we are contracting that goal. >>> We are neither saving memory and at the same time we are not >>> cache coherent. >> >> We use it to reduce guest memory "as possible as we can", which means we >> first have to ensure the functionality so shared mmap should work when >> users call it, then second reduce memory when users use read/write >> (from/to other files). >> >> In cases where users do read/write in most time and calls shared mmap >> sometimes, disabling shared mmap makes this case out of service, but >> with this flag we still reduce memory and the application works. >> >>> >>> IIUC, for virtiofs, you want to use cache=none but at the same time >>> allow guest applications to do shared mmap() hence you are introducing >>> this change. There have been folks who have complained about it >>> and I think 9pfs offers a mode which does this. So solving this >>> problem will be nice. >>> >>> BTW, if "-o dax" is used, it solves this problem. But unfortunately qemu >>> does not have DAX support yet and we also have issues with page >>> truncation >>> on host and MCE not travelling into the guest. So DAX is not a perfect >>> option yet. >> >> Yea, just like I relied in another mail, users' IO pattern may be a >> bunch of small IO to a bunch of small files, dax may help but not so >> much in that case. >> >>> >>> I agree that solving this problem will be nice. Just trying to >>> understand the behavior better. How these cache pages will >>> interact with read/write? >> >> I went through the code, it looks like there are issues when users mmap >> a file and then write to it, this may cause coherency problem between >> the backend file and the frontend page cache. >> I think this problem exists before this patchset: when we private mmap >> a file and then write to it in FOPEN_DIRECT_IO mode, the change doesn't >> update to the page cache because we falsely assume there is no page >> cache under FOPEN_DIRECT_IO mode. I need to go over the code and do some >> test to see if it is really there and to solve it. > > IIUC, I guess the current read/write routine will still initiate DIRECT > IO to server in FOPEN_DIRECT_IO mode, even there's page cache initiated > by shared mmap? Yes, currently no matter we private or shared mmap a file in FOPEN_DIRECT_IO, when we call syscall write to that file, it goes to backend directly, what's worse, it doesn't invalidate the page cache, I've filed a patch for it: https://lore.kernel.org/linux-fsdevel/0625d0cb-2a65-ffae-b072-e14a3f6c7571@linux.dev/ In this patch, I flush pages regardless the mmap is private or shared, that will be tweaked in v2. glad if you have time to reviewing. > > Private mmap doesn't need to care about the coherency issue, as private > mmap is private and doesn't need to be flushed to server. Thus IMHO the Yea, but just like what I said, we should invalidate the page cache page for private mmaped file. > weakened DIRECT_IO, or dio_shared_mmap mode only applies to scenarios > where strong coherency is not needed. > hmmm, not exactly, we should flush the page cache page in that case to enforce strong coherency. Thanks, Hao