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 X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8DDF7C43331 for ; Fri, 6 Sep 2019 13:41:50 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 3005420650 for ; Fri, 6 Sep 2019 13:41:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="osdq9PRz" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3005420650 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 90A1E6B0007; Fri, 6 Sep 2019 09:41:49 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 8934F6B0008; Fri, 6 Sep 2019 09:41:49 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 75BBD6B000A; Fri, 6 Sep 2019 09:41:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0125.hostedemail.com [216.40.44.125]) by kanga.kvack.org (Postfix) with ESMTP id 4F5426B0007 for ; Fri, 6 Sep 2019 09:41:49 -0400 (EDT) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with SMTP id E2550181AC9AE for ; Fri, 6 Sep 2019 13:41:48 +0000 (UTC) X-FDA: 75904608696.30.print71_2cd472d3faa26 X-HE-Tag: print71_2cd472d3faa26 X-Filterd-Recvd-Size: 5464 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) by imf27.hostedemail.com (Postfix) with ESMTP for ; Fri, 6 Sep 2019 13:41:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=eegmsTGmsjmxw5HJRa10m4NP4ml+VRXyFSqt/YxL7Zk=; b=osdq9PRzTHhnM8bKrfYV9bnhT OxjygjzekyVrs9BeOpbFjdiwDpZZGtL3LLo0/So0jLvXEpDA7C5B5mOnPMl5AK7+5wx8hDY7fIL8R W+Z+qvAWQ4sBhQ4KdKV46mZ1hiMSAC+2tfDKwddikTEkf1WZzPQ/+68k0ZWkBo/dJg9HGqLEqRsxd T5pjVdxvTWS8ffeF/XrNGnXyJ3XHSD+8Ylloo1IWuRndopnzrr+epFJmALELupnl/TDhzrvCINFrW cIuCuXQ9+Cnr/iqyxJ7m0sKugEnVCAzgP260WQhjWpOIQAIrLfewbGEZHH01ANfSe8NvsV9mYbrkr QfTk+Wepw==; Received: from willy by bombadil.infradead.org with local (Exim 4.92 #3 (Red Hat Linux)) id 1i6EUj-0003Cg-8a; Fri, 06 Sep 2019 13:41:45 +0000 Date: Fri, 6 Sep 2019 06:41:45 -0700 From: Matthew Wilcox To: "Kirill A. Shutemov" Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Song Liu , William Kucharski , Johannes Weiner Subject: Re: [PATCH 3/3] mm: Allow find_get_page to be used for large pages Message-ID: <20190906134145.GW29434@bombadil.infradead.org> References: <20190905182348.5319-1-willy@infradead.org> <20190905182348.5319-4-willy@infradead.org> <20190906125928.urwopgpd66qibbil@box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190906125928.urwopgpd66qibbil@box> User-Agent: Mutt/1.11.4 (2019-03-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Sep 06, 2019 at 03:59:28PM +0300, Kirill A. Shutemov wrote: > > @@ -248,6 +248,15 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping, > > #define FGP_NOFS 0x00000010 > > #define FGP_NOWAIT 0x00000020 > > #define FGP_FOR_MMAP 0x00000040 > > +/* > > + * If you add more flags, increment FGP_ORDER_SHIFT (no further than 25). > > Maybe some BUILD_BUG_ON()s to ensure FGP_ORDER_SHIFT is sane? Yeah, probably a good idea. > > +/** > > + * __find_get_page - Find and get a page cache entry. > > + * @mapping: The address_space to search. > > + * @offset: The page cache index. > > + * @order: The minimum order of the entry to return. > > + * > > + * Looks up the page cache entries at @mapping between @offset and > > + * @offset + 2^@order. If there is a page cache page, it is returned with > > Off by one? :P Hah! I thought it reasonable to be ambiguous in the English description ... it's not entirely uncommon to describe something being 'between A and B' when meaning ">= A and < B". > > +static struct page *__find_get_page(struct address_space *mapping, > > + unsigned long offset, unsigned int order) > > +{ > > + XA_STATE(xas, &mapping->i_pages, offset); > > + struct page *page; > > + > > + rcu_read_lock(); > > +repeat: > > + xas_reset(&xas); > > + page = xas_find(&xas, offset | ((1UL << order) - 1)); > > Hm. '|' is confusing. What is expectation about offset? > Is round_down(offset, 1UL << order) expected to be equal offset? > If yes, please use '+' instead of '|'. Might make sense to put in ... VM_BUG_ON(offset & ((1UL << order) - 1)); > > + if (xas_retry(&xas, page)) > > + goto repeat; > > + /* > > + * A shadow entry of a recently evicted page, or a swap entry from > > + * shmem/tmpfs. Skip it; keep looking for pages. > > + */ > > + if (xa_is_value(page)) > > + goto repeat; > > + if (!page) > > + goto out; > > + if (compound_order(page) < order) { > > + page = XA_RETRY_ENTRY; > > + goto out; > > + } > > compound_order() is not stable if you don't have pin on the page. > Check it after page_cache_get_speculative(). Maybe check both before and after? If we check it before, we don't bother to bump the refcount on a page which is too small. > > @@ -1632,6 +1696,10 @@ EXPORT_SYMBOL(find_lock_entry); > > * - FGP_FOR_MMAP: Similar to FGP_CREAT, only we want to allow the caller to do > > * its own locking dance if the page is already in cache, or unlock the page > > * before returning if we had to add the page to pagecache. > > + * - FGP_PMD: We're only interested in pages at PMD granularity. If there > > + * is no page here (and FGP_CREATE is set), we'll create one large enough. > > + * If there is a smaller page in the cache that overlaps the PMD page, we > > + * return %NULL and do not attempt to create a page. > > Is it really the best inteface? > > Maybe allow user to ask bitmask of allowed orders? For THP order-0 is fine > if order-9 has failed. That's the semantics that filemap_huge_fault() wants. If the page isn't available at order-9, it needs to return VM_FAULT_FALLBACK (and the VM will call into filemap_fault() to handle the regular sized fault). Now, maybe there are other users who want to specify "create a page of this size if you can, but if there's already something there smaller, return that". We can add another FGP flag when those show up ;-) Thanks for the review.