From patchwork Tue Jan 18 13:54:44 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Howells X-Patchwork-Id: 12716412 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 888D6C433F5 for ; Tue, 18 Jan 2022 13:55:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244356AbiARNzW (ORCPT ); Tue, 18 Jan 2022 08:55:22 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:37361 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244484AbiARNzJ (ORCPT ); Tue, 18 Jan 2022 08:55:09 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1642514109; h=from:from: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:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=VF4onKqMIrYu5mloBKsdeoO9FD6g9xK5Udf/2HJaEOo=; b=gqvn1Nv0w+8HetegEP8ZVF7uSVN5MsWjWyxJWOFrZL4SDfEr5Mzr8S+GzXNnIqVUGp0Wc1 TtGNYI4EiED0dtNF2FkiZVDlDWdikR8lVCz42Y173JcrXhi+pGoLgtxeg6/sDoPA5zZF9Q Sgk6TNn030wRK3LkppyGLYFvLCRmG8k= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-58-vgBLGJ2uORa7FAsl4Z7w7Q-1; Tue, 18 Jan 2022 08:55:03 -0500 X-MC-Unique: vgBLGJ2uORa7FAsl4Z7w7Q-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3A5521090017; Tue, 18 Jan 2022 13:54:49 +0000 (UTC) Received: from warthog.procyon.org.uk (unknown [10.33.36.165]) by smtp.corp.redhat.com (Postfix) with ESMTP id AAE842DE92; Tue, 18 Jan 2022 13:54:45 +0000 (UTC) Organization: Red Hat UK Ltd. 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Registered in England and Wales under Company Registration No. 3798903 Subject: [PATCH 08/11] fscache: Add a comment explaining how page-release optimisation works From: David Howells To: linux-cachefs@redhat.com Cc: Jeff Layton , dhowells@redhat.com, Trond Myklebust , Anna Schumaker , Steve French , Dominique Martinet , Jeff Layton , Matthew Wilcox , Alexander Viro , Omar Sandoval , JeffleXu , Linus Torvalds , linux-afs@lists.infradead.org, linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org, ceph-devel@vger.kernel.org, v9fs-developer@lists.sourceforge.net, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Date: Tue, 18 Jan 2022 13:54:44 +0000 Message-ID: <164251408479.3435901.9540165422908194636.stgit@warthog.procyon.org.uk> In-Reply-To: <164251396932.3435901.344517748027321142.stgit@warthog.procyon.org.uk> References: <164251396932.3435901.344517748027321142.stgit@warthog.procyon.org.uk> User-Agent: StGit/0.23 MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: linux-nfs@vger.kernel.org Add a comment into fscache_note_page_release() to explain how the page-release optimisation logic works[1]. It's not entirely obvious as it has nothing to do with whether or not the netfs file contains data. FSCACHE_COOKIE_NO_DATA_TO_READ is set if we have no data in the cache yet (ie. the backing file lookup was negative, the file is 0 length or the cookie got invalidated). It means that we have no data in the cache, not that the file is necessarily empty on the server. FSCACHE_COOKIE_HAVE_DATA is set once we've stored data in the backing file. From that point on, we have data we *could* read - however, it's covered by pages in the netfs pagecache until at such time one of those covering pages is released. So if we've written data to the cache (HAVE_DATA) and there wasn't any data in the cache when we started (NO_DATA_TO_READ), it may no longer be true that we can skip reading from the cache. Read skipping is done by cachefiles_prepare_read(). Note that tracking is not done on a per-page basis, but only on a per-file basis. Signed-off-by: David Howells Reviewed-by: Jeff Layton cc: linux-cachefs@redhat.com Link: https://lore.kernel.org/r/043a206f03929c2667a465314144e518070a9b2d.camel@kernel.org/ [1] --- include/linux/fscache.h | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/include/linux/fscache.h b/include/linux/fscache.h index ede50406bcb0..296c5f1d9f35 100644 --- a/include/linux/fscache.h +++ b/include/linux/fscache.h @@ -665,6 +665,11 @@ static inline void fscache_clear_inode_writeback(struct fscache_cookie *cookie, static inline void fscache_note_page_release(struct fscache_cookie *cookie) { + /* If we've written data to the cache (HAVE_DATA) and there wasn't any + * data in the cache when we started (NO_DATA_TO_READ), it may no + * longer be true that we can skip reading from the cache - so clear + * the flag that causes reads to be skipped. + */ if (cookie && test_bit(FSCACHE_COOKIE_HAVE_DATA, &cookie->flags) && test_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags))