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 2D836C4332F for ; Mon, 12 Dec 2022 16:31:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232579AbiLLQbb (ORCPT ); Mon, 12 Dec 2022 11:31:31 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36710 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232286AbiLLQbY (ORCPT ); Mon, 12 Dec 2022 11:31:24 -0500 Received: from mail-wm1-x333.google.com (mail-wm1-x333.google.com [IPv6:2a00:1450:4864:20::333]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 18A31FCEC for ; Mon, 12 Dec 2022 08:31:21 -0800 (PST) Received: by mail-wm1-x333.google.com with SMTP id n9-20020a05600c3b8900b003d0944dba41so5669893wms.4 for ; Mon, 12 Dec 2022 08:31:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:mime-version:content-transfer-encoding:fcc:subject:date:from :references:in-reply-to:message-id:from:to:cc:subject:date :message-id:reply-to; bh=Ed5q4EI9Rja0zU3IUSIZ9CyNU+B6k57eyHEpfMq9F2s=; b=N9TLo4b0YJAnMqBn7W6Xr+yD4fMQjYFNCu4raw++ZuwTHvvqA8/fRckg1VsCsjkvSc vQ+Y52KRXPeEiYK1FWSW6PhHocXMZNG6vBLjTBK4rNHmskZ+rSN2C82gePUYo4Zi2BAk xByf+GFGHmSgnxmPlXi4+uDqhzOlEPFtl36OEEZqwbbcWH3lQkUyDwfoCM5dEKjLN1eK ZI2T0J5PVBw7TYM5pG+LcvWaCg9YdjZszYCRJ5zd2vSZLXQqrJw8/HrcZHMfq9rLY2jL JovkWzR3WZpkd/u4KPX6zjJLvsR453iYN85wgtWEmDMkfBQ7faIEu66l9T7cEM9K2WnZ YZdg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:mime-version:content-transfer-encoding:fcc:subject:date:from :references:in-reply-to:message-id:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=Ed5q4EI9Rja0zU3IUSIZ9CyNU+B6k57eyHEpfMq9F2s=; b=D70omHTwMVTLINh16tOI02xENKmNRvWGjK9iQ8WndjoGbnE2tCurVdhMAjcmEty4EI Ryd9f7DK3UOcEwHSPLp5dxjI6GUfVGI/raTAtmtKT0aapatOdXI8fNNoI6zER6Jxg+aY /8NYBzeEXrVBw8uvZx5UNjxmg6XjCN/KcB/Yf0+8TDWJgtB8vntv17p5iIRd8hsSnAuy /DFMJsEiyVKQq0KE+NzIKUTuMK3l7U4eu4L831jgjG9wrYC/Afj/v5EYhyVX8c2EDCT/ 9OCKbb87XPPuK6k7F98f5P7AQ3H+8Uw/Vik7T3hf9bzoLbZNVWcUj9tUWZgDdLbWiJRL hrQw== X-Gm-Message-State: ANoB5plzmDfCH3dvT1GGj61JyquMka7fu4T5bSFacv2xl32MiA9fss8a NvwQTOVfzdTptYO6h5xfjoEh/qgW8PY= X-Google-Smtp-Source: AA0mqf797uGpLey11OSWDFflIbPaz21DA48ut5R8Jh7CP6rz6hlS8Gk+Xw2l6RRljzE0H5ifCYTQsw== X-Received: by 2002:a05:600c:2201:b0:3cf:6be3:a7f6 with SMTP id z1-20020a05600c220100b003cf6be3a7f6mr13007975wml.13.1670862679488; Mon, 12 Dec 2022 08:31:19 -0800 (PST) Received: from [127.0.0.1] ([13.74.141.28]) by smtp.gmail.com with ESMTPSA id n12-20020a05600c3b8c00b003cfd10a33afsm10700061wms.11.2022.12.12.08.31.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 12 Dec 2022 08:31:19 -0800 (PST) Message-Id: In-Reply-To: References: From: "Derrick Stolee via GitGitGadget" Date: Mon, 12 Dec 2022 16:31:14 +0000 Subject: [PATCH v2 1/4] hashfile: allow skipping the hash function Fcc: Sent Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit MIME-Version: 1.0 To: git@vger.kernel.org Cc: gitster@pobox.com, vdye@github.com, avarab@gmail.com, newren@gmail.com, Derrick Stolee , Derrick Stolee Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org From: Derrick Stolee The hashfile API is useful for generating files that include a trailing hash of the file's contents up to that point. Using such a hash is helpful for verifying the file for corruption-at-rest, such as a faulty drive causing flipped bits. Git's index file includes this trailing hash, so it uses a 'struct hashfile' to handle the I/O to the file. This was very convenient to allow using the hashfile methods during these operations. However, hashing the file contents during write comes at a performance penalty. It's slower to hash the bytes on their way to the disk than without that step. This problem is made worse by the replacement of hardware-accelerated SHA1 computations with the software-based sha1dc computation. This write cost is significant, and the checksum capability is likely not worth that cost for such a short-lived file. The index is rewritten frequently and the only time the checksum is checked is during 'git fsck'. Thus, it would be helpful to allow a user to opt-out of the hash computation. We first need to allow Git to opt-out of the hash computation in the hashfile API. The buffered writes of the API are still helpful, so it makes sense to make the change here. Introduce a new 'skip_hash' option to 'struct hashfile'. When set, the update_fn and final_fn members of the_hash_algo are skipped. When finalizing the hashfile, the trailing hash is replaced with the null hash. This use of a trailing null hash would be desireable in either case, since we do not want to special case a file format to have a different length depending on whether it was hashed or not. When the final bytes of a file are all zero, we can infer that it was written without hashing, and thus that verification is not available as a check for file consistency. This also means that we could easily toggle hashing for any file format we desire. A version of this patch has existed in the microsoft/git fork since 2017 [1] (the linked commit was rebased in 2018, but the original dates back to January 2017). Here, the change to make the index use this fast path is delayed until a later change. [1] https://github.com/microsoft/git/commit/21fed2d91410f45d85279467f21d717a2db45201 Co-authored-by: Kevin Willford Signed-off-by: Kevin Willford Signed-off-by: Derrick Stolee --- csum-file.c | 14 +++++++++++--- csum-file.h | 7 +++++++ 2 files changed, 18 insertions(+), 3 deletions(-) diff --git a/csum-file.c b/csum-file.c index 59ef3398ca2..cce13c0f047 100644 --- a/csum-file.c +++ b/csum-file.c @@ -45,7 +45,8 @@ void hashflush(struct hashfile *f) unsigned offset = f->offset; if (offset) { - the_hash_algo->update_fn(&f->ctx, f->buffer, offset); + if (!f->skip_hash) + the_hash_algo->update_fn(&f->ctx, f->buffer, offset); flush(f, f->buffer, offset); f->offset = 0; } @@ -64,7 +65,12 @@ int finalize_hashfile(struct hashfile *f, unsigned char *result, int fd; hashflush(f); - the_hash_algo->final_fn(f->buffer, &f->ctx); + + if (f->skip_hash) + hashclr(f->buffer); + else + the_hash_algo->final_fn(f->buffer, &f->ctx); + if (result) hashcpy(result, f->buffer); if (flags & CSUM_HASH_IN_STREAM) @@ -108,7 +114,8 @@ void hashwrite(struct hashfile *f, const void *buf, unsigned int count) * the hashfile's buffer. In this block, * f->offset is necessarily zero. */ - the_hash_algo->update_fn(&f->ctx, buf, nr); + if (!f->skip_hash) + the_hash_algo->update_fn(&f->ctx, buf, nr); flush(f, buf, nr); } else { /* @@ -153,6 +160,7 @@ static struct hashfile *hashfd_internal(int fd, const char *name, f->tp = tp; f->name = name; f->do_crc = 0; + f->skip_hash = 0; the_hash_algo->init_fn(&f->ctx); f->buffer_len = buffer_len; diff --git a/csum-file.h b/csum-file.h index 0d29f528fbc..29468067f81 100644 --- a/csum-file.h +++ b/csum-file.h @@ -20,6 +20,13 @@ struct hashfile { size_t buffer_len; unsigned char *buffer; unsigned char *check_buffer; + + /** + * If set to 1, skip_hash indicates that we should + * not actually compute the hash for this hashfile and + * instead only use it as a buffered write. + */ + unsigned int skip_hash; }; /* Checkpoint */ -- gitgitgadget