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 X-Spam-Level: X-Spam-Status: No, score=-6.6 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BEEE6C433E3 for ; Tue, 25 Aug 2020 08:25:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8AD322071E for ; Tue, 25 Aug 2020 08:25:22 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="hvQBJRIH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728860AbgHYIZV (ORCPT ); Tue, 25 Aug 2020 04:25:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60354 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728093AbgHYIZT (ORCPT ); Tue, 25 Aug 2020 04:25:19 -0400 Received: from mail-io1-xd44.google.com (mail-io1-xd44.google.com [IPv6:2607:f8b0:4864:20::d44]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AE4EBC061574; Tue, 25 Aug 2020 01:25:19 -0700 (PDT) Received: by mail-io1-xd44.google.com with SMTP id d18so2269719iop.13; Tue, 25 Aug 2020 01:25:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=PJ/LR4qMx00hGSLfoeaJRDPWjskXNla6H/yFj2rArUM=; b=hvQBJRIHHRWicS4maCvOJT+dS0XPa/SgUHCTy5k1iO3EDYu1e3v8eEkLoZIFK0GGrq 1uBQZL/HTbPW5o97QMkIoz7Ut3nppenZM362H7vJBHHpCwfnR1bR5mRlfWKll8tXjLtD N56KBOQHGuPAaJAhmTyJZbhzhJinxi+WMvXafNBh2L0klkuumLHnKGwAXp0xHDZUD+DX dhWgt0DO44OlpI3XnQQd4IDJHEpQsmIUHG5vRPSuCNAcjPF51O5C8tO3JPAFJCcrb2Xc /HbxN2p/gv7GfaAddoxbI6yLmyiwPDDim0Qxqp4I1V3HDWyfxTMrZtRiHJkjbN4NKnCX i5wA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=PJ/LR4qMx00hGSLfoeaJRDPWjskXNla6H/yFj2rArUM=; b=XI5jfUNdzpn4uA8Flm9BxoBSgn91gzyiYpho1zdUYmvqOQROLLIptu6vW0b8nIycmD fSDt3mVQs7q/5F/Am2vw5QlTU+gPCJpt25b+Gtoyof9pYN95iNvvzI9cRDGpTwya1y/T 5nKWvOLAGVLG/ClW+0kZL1iG7pDg3oSW89QZeqiuc6rfzO7XRD3ZUYNUsXzpGdlOhzWi uwBQ2cXLjGhWhdLwty/bD3zEPSBWv6v3+Ez4kMrNn3frXCc/tLGLsottZ1rEn4MPIixy 8Zb+7wd6bsrWEYwspl8q2z3JOFRNJFHGT7ETcO0+NZH/cRdBME0/RG6jZvYv7K4zmd5L +smw== X-Gm-Message-State: AOAM532YoYQ9WLa/Ona4HwftYR7iaEBmSaQaRtEnXGdjMK4YHIYRReOt /agcyfExGbK2knWnGFEKckqzd6NAgCS2GX8G0To= X-Google-Smtp-Source: ABdhPJzZnWiYV7v06E1+iBPLvYsLAKTDeVRUxo/cuk1EqnX4itnv70NikJmR4Z9vjRNFI+99vwaDOJ/Lvx89TntDuDA= X-Received: by 2002:a02:3f2d:: with SMTP id d45mr827548jaa.120.1598343916762; Tue, 25 Aug 2020 01:25:16 -0700 (PDT) MIME-Version: 1.0 References: <9020a583581b644ae86b7c05de6a39fd5204f06d.1597993855.git.osandov@osandov.com> <20200824234903.GA202819@exodia.localdomain> In-Reply-To: <20200824234903.GA202819@exodia.localdomain> From: Amir Goldstein Date: Tue, 25 Aug 2020 11:25:05 +0300 Message-ID: Subject: Re: [PATCH v5 3/9] fs: add RWF_ENCODED for reading/writing compressed data To: Omar Sandoval Cc: linux-fsdevel , Linux Btrfs , Al Viro , Christoph Hellwig , Dave Chinner , Jann Horn , Aleksa Sarai , Linux API , kernel-team@fb.com Content-Type: text/plain; charset="UTF-8" Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Tue, Aug 25, 2020 at 2:49 AM Omar Sandoval wrote: > > On Fri, Aug 21, 2020 at 11:47:54AM +0300, Amir Goldstein wrote: > > On Fri, Aug 21, 2020 at 10:38 AM Omar Sandoval wrote: > > > > > > From: Omar Sandoval > > > > > > Btrfs supports transparent compression: data written by the user can be > > > compressed when written to disk and decompressed when read back. > > > However, we'd like to add an interface to write pre-compressed data > > > directly to the filesystem, and the matching interface to read > > > compressed data without decompressing it. This adds support for > > > so-called "encoded I/O" via preadv2() and pwritev2(). > > > > > > A new RWF_ENCODED flags indicates that a read or write is "encoded". If > > > this flag is set, iov[0].iov_base points to a struct encoded_iov which > > > is used for metadata: namely, the compression algorithm, unencoded > > > (i.e., decompressed) length, and what subrange of the unencoded data > > > should be used (needed for truncated or hole-punched extents and when > > > reading in the middle of an extent). For reads, the filesystem returns > > > this information; for writes, the caller provides it to the filesystem. > > > iov[0].iov_len must be set to sizeof(struct encoded_iov), which can be > > > used to extend the interface in the future a la copy_struct_from_user(). > > > The remaining iovecs contain the encoded extent. > > > > > > This adds the VFS helpers for supporting encoded I/O and documentation > > > for filesystem support. > > > > > > Signed-off-by: Omar Sandoval > > > --- > > > Documentation/filesystems/encoded_io.rst | 74 ++++++++++ > > > Documentation/filesystems/index.rst | 1 + > > > include/linux/fs.h | 16 +++ > > > include/uapi/linux/fs.h | 33 ++++- > > > mm/filemap.c | 166 +++++++++++++++++++++-- > > > 5 files changed, 276 insertions(+), 14 deletions(-) > > > create mode 100644 Documentation/filesystems/encoded_io.rst > > > > > > diff --git a/Documentation/filesystems/encoded_io.rst b/Documentation/filesystems/encoded_io.rst > > > new file mode 100644 > > > index 000000000000..50405276d866 > > > --- /dev/null > > > +++ b/Documentation/filesystems/encoded_io.rst > > > @@ -0,0 +1,74 @@ > > > +=========== > > > +Encoded I/O > > > +=========== > > > + > > > +Encoded I/O is a mechanism for reading and writing encoded (e.g., compressed > > > +and/or encrypted) data directly from/to the filesystem. The userspace interface > > > +is thoroughly described in the :manpage:`encoded_io(7)` man page; this document > > > +describes the requirements for filesystem support. > > > + > > > +First of all, a filesystem supporting encoded I/O must indicate this by setting > > > +the ``FMODE_ENCODED_IO`` flag in its ``file_open`` file operation:: > > > + > > > + static int foo_file_open(struct inode *inode, struct file *filp) > > > + { > > > + ... > > > + filep->f_mode |= FMODE_ENCODED_IO; > > > + ... > > > + } > > > + > > > +Encoded I/O goes through ``read_iter`` and ``write_iter``, designated by the > > > +``IOCB_ENCODED`` flag in ``kiocb->ki_flags``. > > > + > > > +Reads > > > +===== > > > + > > > +Encoded ``read_iter`` should: > > > + > > > +1. Call ``generic_encoded_read_checks()`` to validate the file and buffers > > > + provided by userspace. > > > +2. Initialize the ``encoded_iov`` appropriately. > > > +3. Copy it to the user with ``copy_encoded_iov_to_iter()``. > > > +4. Copy the encoded data to the user. > > > +5. Advance ``kiocb->ki_pos`` by ``encoded_iov->len``. > > > +6. Return the size of the encoded data read, not including the ``encoded_iov``. > > > + > > > +There are a few details to be aware of: > > > + > > > +* Encoded ``read_iter`` should support reading unencoded data if the extent is > > > + not encoded. > > > +* If the buffers provided by the user are not large enough to contain an entire > > > + encoded extent, then ``read_iter`` should return ``-ENOBUFS``. This is to > > > + avoid confusing userspace with truncated data that cannot be properly > > > + decoded. > > > +* Reads in the middle of an encoded extent can be returned by setting > > > + ``encoded_iov->unencoded_offset`` to non-zero. > > > +* Truncated unencoded data (e.g., because the file does not end on a block > > > + boundary) may be returned by setting ``encoded_iov->len`` to a value smaller > > > + value than ``encoded_iov->unencoded_len - encoded_iov->unencoded_offset``. > > > + > > > +Writes > > > +====== > > > + > > > +Encoded ``write_iter`` should (in addition to the usual accounting/checks done > > > +by ``write_iter``): > > > + > > > +1. Call ``copy_encoded_iov_from_iter()`` to get and validate the > > > + ``encoded_iov``. > > > +2. Call ``generic_encoded_write_checks()`` instead of > > > + ``generic_write_checks()``. > > > +3. Check that the provided encoding in ``encoded_iov`` is supported. > > > +4. Advance ``kiocb->ki_pos`` by ``encoded_iov->len``. > > > +5. Return the size of the encoded data written. > > > + > > > +Again, there are a few details: > > > + > > > +* Encoded ``write_iter`` doesn't need to support writing unencoded data. > > > +* ``write_iter`` should either write all of the encoded data or none of it; it > > > + must not do partial writes. > > > +* ``write_iter`` doesn't need to validate the encoded data; a subsequent read > > > + may return, e.g., ``-EIO`` if the data is not valid. > > > +* The user may lie about the unencoded size of the data; a subsequent read > > > + should truncate or zero-extend the unencoded data rather than returning an > > > + error. > > > +* Be careful of page cache coherency. > > > > Haha that rings in my head like the "Smoking kills!" warnings... > > > > I find it a bit odd that you mix page cache at all when reading > > unencoded extents. > > Feels like a file with FMODE_ENCODED_IO should stick to direct IO in all cases. > > I don't know how btrfs deals with mixing direct IO and page cache IO normally, > > but surely the rules could be made even stricter for an inode accessed with this > > new API? > > > > Is there something I am misunderstanding? > > > > Thanks, > > Amir. > > I'm not completely following here, are you suggesting that if a file is > open with O_ALLOW_ENCODED, buffered I/O to that file should return an > error? No. I don't. > Btrfs at least does the necessary range locking and page cache > invalidation to ensure that direct I/O gets along with buffered I/O (and > now encoded I/O). That's a good start :-) I saw btrfs_encoded_read_regular_fill_pages() and concluded that even in FMODE_ENCODED_IO, when reading an unencoded extent, you fill page cache with the unencoded data. Is that correct? or did I miss read the code? If correct, does it serve any purpose? Seems more sensible to me to read/write FMODE_ENCODED_IO only in direct io regardless if the extent is encoded or not (for simpler code if nothing else). Thanks, Amir.