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 2715EC001DF for ; Sun, 30 Jul 2023 18:02:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229608AbjG3SCx (ORCPT ); Sun, 30 Jul 2023 14:02:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54410 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229522AbjG3SCw (ORCPT ); Sun, 30 Jul 2023 14:02:52 -0400 Received: from out-96.mta0.migadu.com (out-96.mta0.migadu.com [IPv6:2001:41d0:1004:224b::60]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 43FE8FB for ; Sun, 30 Jul 2023 11:02:50 -0700 (PDT) Message-ID: <7adaea37-f84f-9415-41fa-53d36833f8f2@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1690740167; 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=8HJc61IqdhGm/da/cQaPPQOiZNCxMs1+dj7U6lcQ7fI=; b=FCtfYps2vJGPxKh/uVtEYz8f9TP14mQ/mEvUrx4LdWmiv9Obssmu3YpQSTgXXzD1y72p8n oth7IJLtU7Nt3QIx5RkhEdQ2xmO5wZhyry3l4srXeUtF7/6lj+0YvRRtkMzgeooesE+fnq ylUtZm/uYH0FO5crP/EfF4cZxH83GSQ= Date: Mon, 31 Jul 2023 02:02:25 +0800 MIME-Version: 1.0 Subject: Re: [PATCH 3/5] io_uring: add support for getdents Content-Language: en-US To: Christian Brauner , djwong@kernel.org, Dave Chinner , Jens Axboe Cc: io-uring@vger.kernel.org, Dominique Martinet , Pavel Begunkov , Alexander Viro , Stefan Roesch , Clay Harris , linux-fsdevel@vger.kernel.org, Wanpeng Li References: <20230718132112.461218-1-hao.xu@linux.dev> <20230718132112.461218-4-hao.xu@linux.dev> <20230726-leinen-basisarbeit-13ae322690ff@brauner> <20230727-salbe-kurvigen-31b410c07bb9@brauner> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Xu In-Reply-To: <20230727-salbe-kurvigen-31b410c07bb9@brauner> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org Hi Christian, On 7/27/23 22:27, Christian Brauner wrote: > On Thu, Jul 27, 2023 at 07:51:19PM +0800, Hao Xu wrote: >> On 7/26/23 23:00, Christian Brauner wrote: >>> On Tue, Jul 18, 2023 at 09:21:10PM +0800, Hao Xu wrote: >>>> From: Hao Xu >>>> >>>> This add support for getdents64 to io_uring, acting exactly like the >>>> syscall: the directory is iterated from it's current's position as >>>> stored in the file struct, and the file's position is updated exactly as >>>> if getdents64 had been called. >>>> >>>> For filesystems that support NOWAIT in iterate_shared(), try to use it >>>> first; if a user already knows the filesystem they use do not support >>>> nowait they can force async through IOSQE_ASYNC in the sqe flags, >>>> avoiding the need to bounce back through a useless EAGAIN return. >>>> >>>> Co-developed-by: Dominique Martinet >>>> Signed-off-by: Dominique Martinet >>>> Signed-off-by: Hao Xu >>>> --- >>>> include/uapi/linux/io_uring.h | 7 +++++ >>>> io_uring/fs.c | 55 +++++++++++++++++++++++++++++++++++ >>>> io_uring/fs.h | 3 ++ >>>> io_uring/opdef.c | 8 +++++ >>>> 4 files changed, 73 insertions(+) >>>> >>>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h >>>> index 36f9c73082de..b200b2600622 100644 >>>> --- a/include/uapi/linux/io_uring.h >>>> +++ b/include/uapi/linux/io_uring.h >>>> @@ -65,6 +65,7 @@ struct io_uring_sqe { >>>> __u32 xattr_flags; >>>> __u32 msg_ring_flags; >>>> __u32 uring_cmd_flags; >>>> + __u32 getdents_flags; >>>> }; >>>> __u64 user_data; /* data to be passed back at completion time */ >>>> /* pack this to avoid bogus arm OABI complaints */ >>>> @@ -235,6 +236,7 @@ enum io_uring_op { >>>> IORING_OP_URING_CMD, >>>> IORING_OP_SEND_ZC, >>>> IORING_OP_SENDMSG_ZC, >>>> + IORING_OP_GETDENTS, >>>> /* this goes last, obviously */ >>>> IORING_OP_LAST, >>>> @@ -273,6 +275,11 @@ enum io_uring_op { >>>> */ >>>> #define SPLICE_F_FD_IN_FIXED (1U << 31) /* the last bit of __u32 */ >>>> +/* >>>> + * sqe->getdents_flags >>>> + */ >>>> +#define IORING_GETDENTS_REWIND (1U << 0) >>>> + >>>> /* >>>> * POLL_ADD flags. Note that since sqe->poll_events is the flag space, the >>>> * command flags for POLL_ADD are stored in sqe->len. >>>> diff --git a/io_uring/fs.c b/io_uring/fs.c >>>> index f6a69a549fd4..480f25677fed 100644 >>>> --- a/io_uring/fs.c >>>> +++ b/io_uring/fs.c >>>> @@ -47,6 +47,13 @@ struct io_link { >>>> int flags; >>>> }; >>>> +struct io_getdents { >>>> + struct file *file; >>>> + struct linux_dirent64 __user *dirent; >>>> + unsigned int count; >>>> + int flags; >>>> +}; >>>> + >>>> int io_renameat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe) >>>> { >>>> struct io_rename *ren = io_kiocb_to_cmd(req, struct io_rename); >>>> @@ -291,3 +298,51 @@ void io_link_cleanup(struct io_kiocb *req) >>>> putname(sl->oldpath); >>>> putname(sl->newpath); >>>> } >>>> + >>>> +int io_getdents_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe) >>>> +{ >>>> + struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents); >>>> + >>>> + if (READ_ONCE(sqe->off) != 0) >>>> + return -EINVAL; >>>> + >>>> + gd->dirent = u64_to_user_ptr(READ_ONCE(sqe->addr)); >>>> + gd->count = READ_ONCE(sqe->len); >>>> + >>>> + return 0; >>>> +} >>>> + >>>> +int io_getdents(struct io_kiocb *req, unsigned int issue_flags) >>>> +{ >>>> + struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents); >>>> + struct file *file = req->file; >>>> + unsigned long getdents_flags = 0; >>>> + bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK; >>> >>> Hm, I'm not sure what exactly the rules are for IO_URING_F_NONBLOCK. >>> But to point this out: >>> >>> vfs_getdents() >>> -> iterate_dir() >>> { >>> if (shared) >>> res = down_read_killable(&inode->i_rwsem); >>> else >>> res = down_write_killable(&inode->i_rwsem); >>> } >>> >>> which means you can still end up sleeping here before you go into a >>> filesystem that does actually support non-waiting getdents. So if you >>> have concurrent operations that grab inode lock (touch, mkdir etc) you >>> can end up sleeping here. >>> >>> Is that intentional or an oversight? If the former can someone please >>> explain the rules and why it's fine in this case? >> >> I actually saw this semaphore, and there is another xfs lock in >> file_accessed >> --> touch_atime >> --> inode_update_time >> --> inode->i_op->update_time == xfs_vn_update_time >> >> Forgot to point them out in the cover-letter..., I didn't modify them >> since I'm not very sure about if we should do so, and I saw Stefan's >> patchset didn't modify them too. >> >> My personnal thinking is we should apply trylock logic for this >> inode->i_rwsem. For xfs lock in touch_atime, we should do that since it >> doesn't make sense to rollback all the stuff while we are almost at the >> end of getdents because of a lock. > > That manoeuvres around the problem. Which I'm slightly more sensitive > too as this review is a rather expensive one. > > Plus, it seems fixable in at least two ways: > > For both we need to be able to tell the filesystem that a nowait atime > update is requested. Simple thing seems to me to add a S_NOWAIT flag to > file_time_flags and passing that via i_op->update_time() which already > has a flag argument. That would likely also help kiocb_modified(). > > file_accessed() > -> touch_atime() > -> inode_update_time() > -> i_op->update_time == xfs_vn_update_time() > > Then we have two options afaict: > > (1) best-effort atime update > > file_accessed() already has the builtin assumption that updating atime > might fail for other reasons - see the comment in there. So it is > somewhat best-effort already. > > (2) move atime update before calling into filesystem > > If we want to be sure that access time is updated when a readdir request > is issued through io_uring then we need to have file_accessed() give a > return value and expose a new helper for io_uring or modify > vfs_getdents() to do something like: > > vfs_getdents() > { > if (nowait) > down_read_trylock() > > if (!IS_DEADDIR(inode)) { > ret = file_accessed(file); > if (ret == -EAGAIN) > goto out_unlock; > > f_op->iterate_shared() > } > } > > It's not unprecedented to do update atime before the actual operation > has been done afaict. That's already the case in xfs_file_write_checks() > which is called before anything is written. So that seems ok. I'm not familiar with this part(the time update), I guess we should revert the updated time if we succeed to do file_accessed(file) but fail somewhere later in f_op->iterate_shared()? Or is it definitely counted as an "access" as long as we start to call getdents to a file? Thanks, Hao > > Does any of these two options work for the xfs maintainers and Jens?