From patchwork Fri Dec 16 15:31:15 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Derrick Stolee X-Patchwork-Id: 13075127 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 E4A95C001B2 for ; Fri, 16 Dec 2022 15:31:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230292AbiLPPb0 (ORCPT ); Fri, 16 Dec 2022 10:31:26 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51032 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229627AbiLPPbY (ORCPT ); Fri, 16 Dec 2022 10:31:24 -0500 Received: from mail-wr1-x430.google.com (mail-wr1-x430.google.com [IPv6:2a00:1450:4864:20::430]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3B18A120AB for ; Fri, 16 Dec 2022 07:31:23 -0800 (PST) Received: by mail-wr1-x430.google.com with SMTP id w15so2842988wrl.9 for ; Fri, 16 Dec 2022 07:31:23 -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=gAFl3g0ii3MudzNaTXBcOtIccfisGMWtyBiFHC36zl0=; b=LdDrvVqDNEUKvh2qtdLYqtbUM7JxF9U51EuOCbbADNOX9v8WHtlWUnWUNjjuEE2MZI tJFxnhq2pnRygw9VhYD11OoFOzJgXnwOP1B8UlSK989YtdY8eRWHC3pzRn+RFheM4Q+f nlM7FMqNQV1gDUv4UTfABwrplcXtpMit+FlNz+Lo4xpKhXseWba4aKqzOXL3fQajVE4V 6LiMy1OXF1y9dx9c1gAPoVKL/BJrCZj6+Hz/Y1kZGWMpgX5rUpobKWQ4PSODlOwaEe3t 2cmSvT6BkSczgjHO8V+yHFw3nZyP0e97eAIJqes4YEt075euHmUQj/q9eiyEaMIu6zLm mJqg== 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=gAFl3g0ii3MudzNaTXBcOtIccfisGMWtyBiFHC36zl0=; b=hCXwZAa1AmP/sMFRAknalQL0H4uibGcLXT5kpJPafKmQbzXsEe1UugV8guXOXLu9z6 kO2f57AmnH2Plb91tapGvH9ih53jKbx5ESxWt3cYCnJKkcD1Sx8rAfqjpD+Dn2K7pWp+ FmT/s3S/Fkw+HJcCFrGTTnKSUGvA3XT2fS21rDnfAwYxMcWvUxa+jwR/eTg4sezP4FT/ 5UnZtlvqtHD1N8PrCPzz/KZYJyTcpb50slFS+grJeOner3RNud6+SlzJ+YprmytNuYRE QgJGbi+mtMQ98gSySIpkFCMSdZiVYBc7FvUXBkW6zBNXelFEBKQ1LC0u4kGH4Avah0+n 1OZg== X-Gm-Message-State: ANoB5pmP43jL2bEL/22ph1dXWfH2OMz+qslFuZrcTAwe9gjgi5ircLPm EKqUOaQGPMHqWwV0FAp3tRbWsoqp/XE= X-Google-Smtp-Source: AA0mqf4hhbHIaAL0btHpUnpv8Ewk41xhs5y4MNerCtfRcag1Hy+0YtI1L9dUMbePUFZbwktPVKji2g== X-Received: by 2002:a05:6000:81a:b0:242:69f4:cb75 with SMTP id bt26-20020a056000081a00b0024269f4cb75mr21768111wrb.54.1671204681414; Fri, 16 Dec 2022 07:31:21 -0800 (PST) Received: from [127.0.0.1] ([13.74.141.28]) by smtp.gmail.com with ESMTPSA id p13-20020a056000018d00b002422202fa7fsm2527169wrx.39.2022.12.16.07.31.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 16 Dec 2022 07:31:20 -0800 (PST) Message-Id: In-Reply-To: References: Date: Fri, 16 Dec 2022 15:31:15 +0000 Subject: [PATCH v4 1/4] hashfile: allow skipping the hash function Fcc: Sent MIME-Version: 1.0 To: git@vger.kernel.org Cc: gitster@pobox.com, vdye@github.com, avarab@gmail.com, newren@gmail.com, Jacob Keller , Derrick Stolee , Derrick Stolee Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org From: Derrick Stolee 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..793a59da12b 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 non-zero, skip_hash indicates that we should + * not actually compute the hash for this hashfile and + * instead only use it as a buffered write. + */ + int skip_hash; }; /* Checkpoint */