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 86242C7EE2E for ; Tue, 13 Jun 2023 00:42:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236202AbjFMAm6 (ORCPT ); Mon, 12 Jun 2023 20:42:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40114 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232584AbjFMAm5 (ORCPT ); Mon, 12 Jun 2023 20:42:57 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4355B197; Mon, 12 Jun 2023 17:42:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=p7aw0VIsZS7Nht2qNh9pInAE4cDjMXwvVzTJV++TJ18=; b=Oa7yGVO4QLU5iHwKdEBisWwj6g Dfn3h0LAbAx9d1afS+ERlQklZ16sjepLfWcBT6B1hJ2aqQk5VWaPukWSVnnsBILqu+nqVVZI5+PS3 ndVA+tsFlG3jkyT3nlsLyVFgfxyZAIrEL3G18x4v//HlCGZ+EBh0ZuYi+bxEozLcyDLHS8hn0rctM WnN60qur4j292dFhLFsdiEQ+/B+IZ0lpCbPLaMmG9cjlIgLtu5yV3XRHJ1M7EU9Ql+6cBmRHlmqiI fvycyf26CBNVe9+7EARU5x4fYc9oDGEPxvYkk3A2g7rnCLafqpj4BopGF9EXorA9UeXsPxz5+BYAy PQ2AbWTw==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1q8s7X-003L19-3f; Tue, 13 Jun 2023 00:42:51 +0000 Date: Tue, 13 Jun 2023 01:42:51 +0100 From: Matthew Wilcox To: Dave Chinner Cc: linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, Wang Yugui , Christoph Hellwig , "Darrick J . Wong" Subject: Re: [PATCH v3 6/8] filemap: Allow __filemap_get_folio to allocate large folios Message-ID: References: <20230612203910.724378-1-willy@infradead.org> <20230612203910.724378-7-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Tue, Jun 13, 2023 at 08:49:05AM +1000, Dave Chinner wrote: > On Mon, Jun 12, 2023 at 09:39:08PM +0100, Matthew Wilcox (Oracle) wrote: > > Allow callers of __filemap_get_folio() to specify a preferred folio > > order in the FGP flags. This is only honoured in the FGP_CREATE path; > > if there is already a folio in the page cache that covers the index, > > we will return it, no matter what its order is. No create-around is > > attempted; we will only create folios which start at the specified index. > > Unmodified callers will continue to allocate order 0 folios. > ..... > > - /* Init accessed so avoid atomic mark_page_accessed later */ > > - if (fgp_flags & FGP_ACCESSED) > > - __folio_set_referenced(folio); > > + if (!mapping_large_folio_support(mapping)) > > + order = 0; > > + if (order > MAX_PAGECACHE_ORDER) > > + order = MAX_PAGECACHE_ORDER; > > + /* If we're not aligned, allocate a smaller folio */ > > + if (index & ((1UL << order) - 1)) > > + order = __ffs(index); > > If I read this right, if we pass in an unaligned index, we won't get > the size of the folio we ask for? Right. That's implied by (but perhaps not obvious from) the changelog. Folios are always naturally aligned in the file, so an order-4 folio has to start at a multiple of 16. If the index you pass in is not a multiple of 16, we can't create an order-4 folio without starting at an earlier index. For a 4kB block size filesystem, that's what we want. Applications _generally_ don't write backwards, so creating an order-4 folio is just wasting memory. > e.g. if we want an order-4 folio (64kB) because we have a 64kB block > size in the filesystem, then we have to pass in an index that > order-4 aligned, yes? > > I ask this, because the later iomap code that asks for large folios > only passes in "pos >> PAGE_SHIFT" so it looks to me like it won't > allocate large folios for anything other than large folio aligned > writes, even if we need them. > > What am I missing? Perhaps what you're missing is that this isn't trying to solve the problem of supporting a bs > ps filesystem? That's also a worthwhile project, but it's not this project. In fact, I'd say that project is almost orthogonal to this one; for this usage we can always fall back to smaller folios on memory pressure or misalignment. For a bs > ps block device, we have to allocate folios at least as large as the blocksize and cannot fall back to smaller folios. For a bs > ps filesystem on a bdev with bs == ps, we can fall back (as your prototype showed).