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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 966DDC83F01 for ; Sun, 27 Aug 2023 21:45:44 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 0D22F28000E; Sun, 27 Aug 2023 17:45:44 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 082BB8E0001; Sun, 27 Aug 2023 17:45:44 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E8B8428000E; Sun, 27 Aug 2023 17:45:43 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id D72E98E0001 for ; Sun, 27 Aug 2023 17:45:43 -0400 (EDT) Received: from smtpin22.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 6D0F24021C for ; Sun, 27 Aug 2023 21:45:43 +0000 (UTC) X-FDA: 81171216966.22.7492E48 Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [62.89.141.173]) by imf08.hostedemail.com (Postfix) with ESMTP id 79AC616000A for ; Sun, 27 Aug 2023 21:45:41 +0000 (UTC) Authentication-Results: imf08.hostedemail.com; dkim=pass header.d=linux.org.uk header.s=zeniv-20220401 header.b=rwicGoUv; dmarc=pass (policy=none) header.from=zeniv.linux.org.uk; spf=none (imf08.hostedemail.com: domain of viro@ftp.linux.org.uk has no SPF policy when checking 62.89.141.173) smtp.mailfrom=viro@ftp.linux.org.uk ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1693172741; h=from:from:sender:sender: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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=kY05MXjwCqUKuXNwucsgt/McL4F7q7Pfbjm7IIALTG0=; b=P7oQJBKa9qOk9HK6dlwOn8lPuHcNPeWVVQ4rbPuN9ZmTv73xLyxiJqjI481gXf/V/4BeMt VmSR8tVnS+pWbXbPC/rZInYwHvO5Meco/WSzN6ZuqyOKuVx7jl08l1YDCrS0Z8IiXm1tT6 e3p36N4XoKeiwAoOMJ0E4xsJZ1qnbis= ARC-Authentication-Results: i=1; imf08.hostedemail.com; dkim=pass header.d=linux.org.uk header.s=zeniv-20220401 header.b=rwicGoUv; dmarc=pass (policy=none) header.from=zeniv.linux.org.uk; spf=none (imf08.hostedemail.com: domain of viro@ftp.linux.org.uk has no SPF policy when checking 62.89.141.173) smtp.mailfrom=viro@ftp.linux.org.uk ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1693172741; a=rsa-sha256; cv=none; b=ovaIirqo18FcRW4y5VMVKDZbXMnbTzlqYttNatSzHjibKbgoaIVhXuQMGsp40xmMQ8DYnk goWCnM6/Grpz4tx/1GJnY09B3hL6t2HMtdInGoHUN+PM+xc+viF/Ga04mx0FUqVIP7mebE vjrT8lhHxUZOzIK7BjsKK0aPE0m0ox0= DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=kY05MXjwCqUKuXNwucsgt/McL4F7q7Pfbjm7IIALTG0=; b=rwicGoUvL4VgCd8/Qp1uGcvFJj 2egMEWh2qIptB38yz3LkaovCgsrzYd5qWYzYVrLDB0+2cvC8Hq94M95X4EXXJUaYQexvV5ydz/PhQ 3oSRutUhsOumwt9db2so3z+etJXEF+Wi2+NHOmG8HlbMqrIjqtB+TQ9YoLhUuiexvk8+I1LYJ/xSV ZlLyerq6Wcockw2BEhGC0e7r/uJgQmeAavj3yTO2QGyQwB10XYMkDDZ1Bff7wCxS6uqmy7RDsW0tE LxV9EZ6t5yOJ6J5Tl0xTjIsbFSoYFc5//Rfxs/6ERxlTJUc4/DLKribj0NNKKwb+gHZab/RxADkLO htbC578A==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1qaNZO-001P52-2K; Sun, 27 Aug 2023 21:45:18 +0000 Date: Sun, 27 Aug 2023 22:45:18 +0100 From: Al Viro To: Christoph Hellwig Cc: Matthew Wilcox , Jens Axboe , Xiubo Li , Ilya Dryomov , Christian Brauner , Theodore Ts'o , Jaegeuk Kim , Chao Yu , Miklos Szeredi , Andreas Gruenbacher , "Darrick J. Wong" , Trond Myklebust , Anna Schumaker , Damien Le Moal , Andrew Morton , linux-block@vger.kernel.org, ceph-devel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, Hannes Reinecke Subject: Re: [PATCH 03/12] filemap: update ki_pos in generic_perform_write Message-ID: <20230827214518.GU3390869@ZenIV> References: <20230601145904.1385409-1-hch@lst.de> <20230601145904.1385409-4-hch@lst.de> <20230827194122.GA325446@ZenIV> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230827194122.GA325446@ZenIV> X-Rspamd-Queue-Id: 79AC616000A X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: j77xu4ja46azbx67geb38hxbdzx5pjbk X-HE-Tag: 1693172741-72670 X-HE-Meta: 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 7U7lbQ2J 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sun, Aug 27, 2023 at 08:41:22PM +0100, Al Viro wrote: > On Thu, Jun 01, 2023 at 04:58:55PM +0200, Christoph Hellwig wrote: > > All callers of generic_perform_write need to updated ki_pos, move it into > > common code. > > > @@ -4034,7 +4037,6 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from) > > endbyte = pos + status - 1; > > err = filemap_write_and_wait_range(mapping, pos, endbyte); > > if (err == 0) { > > - iocb->ki_pos = endbyte + 1; > > written += status; > > invalidate_mapping_pages(mapping, > > pos >> PAGE_SHIFT, > > @@ -4047,8 +4049,6 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from) > > } > > } else { > > written = generic_perform_write(iocb, from); > > - if (likely(written > 0)) > > - iocb->ki_pos += written; > > } > > out: > > return written ? written : err; > > [another late reply, sorry] > > That part is somewhat fishy - there's a case where you return a positive value > and advance ->ki_pos by more than that amount. I really wonder if all callers > of ->write_iter() are OK with that. Consider e.g. this: > > ssize_t ksys_write(unsigned int fd, const char __user *buf, size_t count) > { > struct fd f = fdget_pos(fd); > ssize_t ret = -EBADF; > > if (f.file) { > loff_t pos, *ppos = file_ppos(f.file); > if (ppos) { > pos = *ppos; > ppos = &pos; > } > ret = vfs_write(f.file, buf, count, ppos); > if (ret >= 0 && ppos) > f.file->f_pos = pos; > fdput_pos(f); > } > > return ret; > } > > ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_t *pos) > { > ssize_t ret; > > if (!(file->f_mode & FMODE_WRITE)) > return -EBADF; > if (!(file->f_mode & FMODE_CAN_WRITE)) > return -EINVAL; > if (unlikely(!access_ok(buf, count))) > return -EFAULT; > > ret = rw_verify_area(WRITE, file, pos, count); > if (ret) > return ret; > if (count > MAX_RW_COUNT) > count = MAX_RW_COUNT; > file_start_write(file); > if (file->f_op->write) > ret = file->f_op->write(file, buf, count, pos); > else if (file->f_op->write_iter) > ret = new_sync_write(file, buf, count, pos); > else > ret = -EINVAL; > if (ret > 0) { > fsnotify_modify(file); > add_wchar(current, ret); > } > inc_syscw(current); > file_end_write(file); > return ret; > } > > static ssize_t new_sync_write(struct file *filp, const char __user *buf, size_t len, loff_t *ppos) > { > struct kiocb kiocb; > struct iov_iter iter; > ssize_t ret; > > init_sync_kiocb(&kiocb, filp); > kiocb.ki_pos = (ppos ? *ppos : 0); > iov_iter_ubuf(&iter, ITER_SOURCE, (void __user *)buf, len); > > ret = call_write_iter(filp, &kiocb, &iter); > BUG_ON(ret == -EIOCBQUEUED); > if (ret > 0 && ppos) > *ppos = kiocb.ki_pos; > return ret; > } > > Suppose ->write_iter() ends up doing returning a positive value smaller than > the increment of kiocb.ki_pos. What do we get? ret is positive, so > kiocb.ki_pos gets copied into *ppos, which is ksys_write's pos and there > we copy it into file->f_pos. > > Is it really OK to have write() return 4096 and advance the file position > by 16K? AFAICS, userland wouldn't get any indication of something > odd going on - just a short write to a regular file, with followup write > of remaining 12K getting quietly written in the range 16K..28K. > > I don't remember what POSIX says about that, but it would qualify as > nasty surprise for any userland program - sure, one can check fsync() > results before closing the sucker and see if everything looks fine, > but the way it's usually discussed could easily lead to assumption that > (synchronous) O_DIRECT writes would not be affected by anything of that > sort. IOW, I suspect that the right thing to do would be something along the lines of direct_write_fallback(): on error revert the ->ki_pos update from buffered write If we fail filemap_write_and_wait_range() on the range the buffered write went into, we only report the "number of bytes which we direct-written", to quote the comment in there. Which is fine, but buffered write has already advanced iocb->ki_pos, so we need to roll that back. Otherwise we end up with e.g. write(2) advancing position by more than the amount it reports having written. Fixes: 182c25e9c157 "filemap: update ki_pos in generic_perform_write" Signed-off-by: Al Viro --- diff --git a/fs/libfs.c b/fs/libfs.c index 5b851315eeed..712c57828c0e 100644 --- a/fs/libfs.c +++ b/fs/libfs.c @@ -1646,6 +1646,7 @@ ssize_t direct_write_fallback(struct kiocb *iocb, struct iov_iter *iter, * We don't know how much we wrote, so just return the number of * bytes which were direct-written */ + iocb->ki_pos -= buffered_written; if (direct_written) return direct_written; return err;