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=-9.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham 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 F2FB5C35247 for ; Mon, 3 Feb 2020 20:44:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BBF4720658 for ; Mon, 3 Feb 2020 20:44:43 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=toxicpanda-com.20150623.gappssmtp.com header.i=@toxicpanda-com.20150623.gappssmtp.com header.b="U5jr+YeP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726930AbgBCUon (ORCPT ); Mon, 3 Feb 2020 15:44:43 -0500 Received: from mail-qk1-f195.google.com ([209.85.222.195]:41151 "EHLO mail-qk1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726278AbgBCUom (ORCPT ); Mon, 3 Feb 2020 15:44:42 -0500 Received: by mail-qk1-f195.google.com with SMTP id u19so7996537qku.8 for ; Mon, 03 Feb 2020 12:44:41 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=toxicpanda-com.20150623.gappssmtp.com; s=20150623; h=from:to:subject:date:message-id:in-reply-to:references:mime-version :content-transfer-encoding; bh=iUnvrD6m4nNHjITULfSvpib82qzgHURQ3/B0vvzqnJ4=; b=U5jr+YePGu4S/4l34n/LlxT4RLpCLtRYm8223V1hFth/4EIgA+O/hvJ8V0BPJHpmqp L4Sy5+ID6fA1CrI5whNAiY2FDwsb4WX9YiyJMnUgQorDBbJxz2FNWc3cQEfgyK9aBb/+ 3J/pXiFvO2RXrSXQE/j86+ZWRW/OTkRfn8z4ExDWd9bMz4Wt3N4GnOOOdkt0wRsyg/Ab 2rj7YE6hpWvmnmhTepEcg2nxlWiycNOaGKFasWHB0J2Y0KrXenqTHdoWoopHphNTs3hR 296AoZ2us3CyHUqygcMSCLTHwct65Uj9UVahff9ieSTMWk5/8B8dwZ/93d2QRNBZoUR7 PmWA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=iUnvrD6m4nNHjITULfSvpib82qzgHURQ3/B0vvzqnJ4=; b=pDCwlqBHug6vPL85NIVu91m8UwN8GrgpxgdoL3BRCLPtmJX+2IzbDsq/NNn35s6ixg 1UUxaRjwhtetMd1uvPKYYwPQhIPQmooSMjmzRR0ilXzi6RIc5ev/rccna5FFghkXWnEn h/7rv8qxK6bba5gINXl6rLR7XD+X6xZgHlqdktthf8gT7OFZdUryovWkead6uyIAhD6U NJdsGIc+4Af8s7QUNE512IF64sYW5amgEY2unItkI9zblQNMBk/b4+Uo2D3CnHu2Vq6J +Ngf1V1IEopRaxNo+MJJhpmXyPXYF55w3JdmPBEpYR+5210SX6gU9604RtBzup4J/g1S 3beQ== X-Gm-Message-State: APjAAAUIpeOPFrNgvQfdKvqnRe15v4C4RYqsfQ6+MaqCyruiozlRhf98 lRTK9iY/ekb14PM2CsaJ3gFWBJZPfh5+LA== X-Google-Smtp-Source: APXvYqydMzvordAPz1aJDsDekGzhknJHTa2d1uj9Z3ExxM0aYgw9Mzg7oBktOHbA/L/q5AlFFF5ziw== X-Received: by 2002:a05:620a:11a1:: with SMTP id c1mr26203480qkk.390.1580762680547; Mon, 03 Feb 2020 12:44:40 -0800 (PST) Received: from localhost ([107.15.81.208]) by smtp.gmail.com with ESMTPSA id h12sm10470569qtn.56.2020.02.03.12.44.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Feb 2020 12:44:39 -0800 (PST) From: Josef Bacik To: linux-btrfs@vger.kernel.org, kernel-team@fb.com Subject: [PATCH 1/3] btrfs: add a comment describing block-rsvs Date: Mon, 3 Feb 2020 15:44:34 -0500 Message-Id: <20200203204436.517473-2-josef@toxicpanda.com> X-Mailer: git-send-email 2.24.1 In-Reply-To: <20200203204436.517473-1-josef@toxicpanda.com> References: <20200203204436.517473-1-josef@toxicpanda.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org This is a giant comment at the top of block-rsv.c describing generally how block rsvs work. It is purely about the block rsv's themselves, and nothing to do with how the actual reservation system works. Signed-off-by: Josef Bacik --- fs/btrfs/block-rsv.c | 81 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 81 insertions(+) diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c index d07bd41a7c1e..54380f477f80 100644 --- a/fs/btrfs/block-rsv.c +++ b/fs/btrfs/block-rsv.c @@ -6,6 +6,87 @@ #include "space-info.h" #include "transaction.h" +/* + * HOW DO BLOCK RSVS WORK + * + * Think of block_rsv's as bucktes for logically grouped reservations. Each + * block_rsv has a ->size and a ->reserved. ->size is how large we want our + * block rsv to be, ->reserved is how much space is currently reserved for + * this block reserve. + * + * ->failfast exists for the truncate case, and is described below. + * + * NORMAL OPERATION + * We determine we need N items of reservation, we use the appropriate + * btrfs_calc*() helper to determine the number of bytes. We call into + * reserve_metadata_bytes() and get our bytes, we then add this space to our + * ->size and our ->reserved. + * + * We go to modify the tree for our operation, we allocate a tree block, which + * calls btrfs_use_block_rsv(), and subtracts nodesize from + * block_rsv->reserved. + * + * We finish our operation, we subtract our original reservation from ->size, + * and then we subtract ->size from ->reserved if there is an excess and free + * the excess back to the space info, by reducing space_info->bytes_may_use by + * the excess amount. + * + * In some cases we may return this excess to the global block reserve or + * delayed refs reserve if either of their ->size is greater than their + * ->reserved. + * + * BLOCK_RSV_TRANS, BLOCK_RSV_DELOPS, BLOCK_RSV_CHUNK + * These behave normally, as described above, just within the confines of the + * lifetime of ther particular operation (transaction for the whole trans + * handle lifetime, for example). + * + * BLOCK_RSV_GLOBAL + * This has existed forever, with diminishing degrees of importance. + * Currently it exists to save us from ourselves. We definitely over-reserve + * space most of the time, but the nature of COW is that we do not know how + * much space we may need to use for any given operation. This is + * particularly true about the extent tree. Modifying one extent could + * balloon into 1000 modifications of the extent tree, which we have no way of + * properly predicting. To cover this case we have the global reserve act as + * the "root" space to allow us to not abort the transaciton when things are + * very tight. As such we tend to treat this space as sacred, and only use it + * if we are desparate. Generally we should no longer be depending on its + * space, and if new use cases arise we need to address them elsewhere. + * + * BLOCK_RSV_DELALLOC + * The individual item sizes are determined by the per-inode size + * calculations, which are described with the delalloc code. This is pretty + * straightforward, it's just the calculation of ->size encodes a lot of + * different items, and thus it gets used when updating inodes, inserting file + * extents, and inserting checksums. + * + * BLOCK_RSV_DELREFS + * We keep a running talley of how many delayed refs we have on the system. + * We assume each one of these delayed refs are going to use a full + * reservation. We use the transaction items and pre-reserve space for every + * operation, and use this reservation to refill any gap between ->size and + * ->reserved that may exist. + * + * From there it's straightforward, removing a delayed ref means we remove its + * count from ->size and free up reservations as necessary. Since this is the + * most dynamic block rsv in the system, we will try to refill this block rsv + * first with any excess returned by any other block reserve. + * + * BLOCK_RSV_EMPTY + * This is the fallback block rsv to make us try to reserve space if we don't + * have a specific bucket for this allocation. It is mostly used for updating + * the device tree and such, since that is a separate pool we're content to + * just reserve space from the space_info on demand. + * + * BLOCK_RSV_TEMP + * This is used by things like truncate and iput. We will temporarily + * allocate a block rsv, set it to some size, and then truncate bytes until we + * have no space left. With ->failfast set we'll simply return ENOSPC from + * btrfs_use_block_rsv() to signal that we need to unwind and try to make a + * new reservation. This is because these operations are unbounded, so we + * want to do as much work as we can, and then back off and re-reserve. + */ + static u64 block_rsv_release_bytes(struct btrfs_fs_info *fs_info, struct btrfs_block_rsv *block_rsv, struct btrfs_block_rsv *dest, u64 num_bytes, -- 2.24.1