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 CF3CBE784AF for ; Mon, 2 Oct 2023 09:55:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236301AbjJBJze (ORCPT ); Mon, 2 Oct 2023 05:55:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46426 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236112AbjJBJz2 (ORCPT ); Mon, 2 Oct 2023 05:55:28 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [IPv6:2001:67c:2178:6::1c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0CFEBA7 for ; Mon, 2 Oct 2023 02:55:25 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id B90F82185A; Mon, 2 Oct 2023 09:55:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1696240523; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=a5qSHb2HY4O8xrSTCIffNnESicw2f6a/GZiMI+Le440=; b=BwyfoNRWkaue7UlzJq8yzWf5se6SWP9HRooAzqRkIdEmCRyV+PDH5v/Y3Gxib+XuSJzGwt N8qAAlg6ZvoYeec2zurYZzNcsY0tY4K6NeG4jsWXkGSGcWDAVXi8L77it3P2fyq+xsvqPA mI+cvpcEbLP3Ktx8QLSOUmW0zNfPqK0= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1696240523; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=a5qSHb2HY4O8xrSTCIffNnESicw2f6a/GZiMI+Le440=; b=FPv/6jZ7LK9Jbe325/Ith2a3jrIj4f1jDpacojNccVHQKwc5wQBlJPuQYC+hRPZ5wO72Wt kHZ2Q9GLb48tCkBg== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id ABF5813434; Mon, 2 Oct 2023 09:55:23 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id ZJPoKYuTGmUXUgAAMHmgww (envelope-from ); Mon, 02 Oct 2023 09:55:23 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 48D3FA07C9; Mon, 2 Oct 2023 11:55:23 +0200 (CEST) Date: Mon, 2 Oct 2023 11:55:23 +0200 From: Jan Kara To: Lorenzo Stoakes Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Matthew Wilcox , Jan Kara Subject: Re: [PATCH] mm/filemap: clarify filemap_fault() comments for not uptodate case Message-ID: <20231002095523.6gttx365tvfaeexb@quack3> References: <20230930231029.88196-1-lstoakes@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230930231029.88196-1-lstoakes@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun 01-10-23 00:10:29, Lorenzo Stoakes wrote: > The existing comments in filemap_fault() suggest that, after either a minor > fault has occurred and filemap_get_folio() found a folio in the page cache, > or a major fault arose and __filemap_get_folio(FGP_CREATE...) did the job > (having relied on do_sync_mmap_readahead() or filemap_read_folio() to read > in the folio), the only possible reason it could not be uptodate is because > of an error. > > This is not so, as if, for instance, the fault occurred within a VMA which > had the VM_RAND_READ flag set (via madvise() with the MADV_RANDOM flag > specified), this would cause even synchronous readahead to fail to read in > the folio. > > I confirmed this by dropping page caches and faulting in memory madvise()'d > this way, observing that this code path was reached on each occasion. > > Clarify the comments to include this case, and additionally update the > comment recently added around the invalidate lock logic to make it clear > the comment explicitly refers to the minor fault case. > > In addition, while we're here, refer to folios rather than pages. > > Signed-off-by: Lorenzo Stoakes After the alignment fixup the patch looks good to me. Feel free to add: Reviewed-by: Jan Kara Honza > --- > mm/filemap.c | 19 +++++++++++++------ > 1 file changed, 13 insertions(+), 6 deletions(-) > > diff --git a/mm/filemap.c b/mm/filemap.c > index d285ec5f9162..959694a2ade7 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -3308,21 +3308,28 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) > VM_BUG_ON_FOLIO(!folio_contains(folio, index), folio); > > /* > - * We have a locked page in the page cache, now we need to check > - * that it's up-to-date. If not, it is going to be due to an error. > + * We have a locked folio in the page cache, now we need to check > + * that it's up-to-date. If not, it is going to be due to an error, > + * or because readahead was otherwise unable to retrieve it. > */ > if (unlikely(!folio_test_uptodate(folio))) { > /* > - * The page was in cache and uptodate and now it is not. > - * Strange but possible since we didn't hold the page lock all > - * the time. Let's drop everything get the invalidate lock and > - * try again. > + * If the invalidate lock is not held, the folio was in cache and > + * uptodate and now it is not. Strange but possible since we > + * didn't hold the page lock all the time. Let's drop everything, > + * get the invalidate lock and try again. > */ > if (!mapping_locked) { > folio_unlock(folio); > folio_put(folio); > goto retry_find; > } > + > + /* > + * OK, the folio is really not uptodate. This can be because the > + * VMA has the VM_RAND_READ flag set, or because an error > + * arose. Let's read it in directly. > + */ > goto page_not_uptodate; > } > > -- > 2.42.0 > -- Jan Kara SUSE Labs, CR