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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id D0EC6C4332F for ; Thu, 2 Nov 2023 12:56:22 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 154578D0055; Thu, 2 Nov 2023 08:56:22 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 104818D000F; Thu, 2 Nov 2023 08:56:22 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F37118D0055; Thu, 2 Nov 2023 08:56:21 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id E26D28D000F for ; Thu, 2 Nov 2023 08:56:21 -0400 (EDT) Received: from smtpin04.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay02.hostedemail.com (Postfix) with ESMTP id A2856120488 for ; Thu, 2 Nov 2023 12:56:21 +0000 (UTC) X-FDA: 81413012562.04.C157884 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf30.hostedemail.com (Postfix) with ESMTP id 2F6BA80011 for ; Thu, 2 Nov 2023 12:56:18 +0000 (UTC) Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=djPoEPuh; dmarc=none; spf=none (imf30.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1698929779; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding:in-reply-to: references:dkim-signature; bh=GgcTWdqaw8oTk+H9w/UodRkn+QGXypNGd40ay+cpcY0=; b=mxhm80QLM5aNJ0l48kR3+TsjHmO11pB7czB7mb4ZBtdrsmBsIQCKctmhZGFsy6EdHASw6Z sgjeAXLjCSt+OFLuQ73zRy/MoJ5/AV8WfgE1xKjyPPsvi2th71gZp2fYYjUcigFSmUm3xj XbEDibimRw6CSV+hk9ZkY0BEazQoeog= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=djPoEPuh; dmarc=none; spf=none (imf30.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1698929779; a=rsa-sha256; cv=none; b=zn3DcEV6sBt1U1SJ2phkDreEGKuoOk77oWVpQufRtoojuDTDR3DBvSGXyu1LFGomJ9GYeF NdYGmFXIRHcw8x8UfB08A0IbKmbpmQynQqc4M7MOAPLdiwPOnN0AWPcwJCjJimm7PQ/zjj BPoFHrJdahVIb7A0xWZOSKujhkicdh0= DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=Content-Type:MIME-Version:Message-ID: Subject:To:From:Date:Sender:Reply-To:Cc:Content-Transfer-Encoding:Content-ID: Content-Description:In-Reply-To:References; bh=GgcTWdqaw8oTk+H9w/UodRkn+QGXypNGd40ay+cpcY0=; b=djPoEPuh+ZLdtsbjdVDzS8CFes b8Z/04IZs0mtZKhqIW/0RO3WsG6AOCROoYP8hZ04nxikF9j/LRA888xmDDtd4bQB9XsKSkQMlr9AT xDUZvte1c2a9XtkE2UlV8HzuHvFEiPMZ2MJF0XRgvMDTobSRQKbCqG4GbI/tZeaQpoODbsLwMxBeG L5B9lCNfhw8hFpIbuTa9IikQu+d4ReMcfBhFR1nENlYfnLsfa8IdnCjHLpx7wcuc4j6zYgF/SDZ8M iWe5IBRRL9kSBTIqpkEdXII6E5z1QzE47l93X6JWvguRQMJ8P3zTjfB2mVAa/cFyJGD3/G0TTlLf9 xM1Yr8/g==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qyXF9-004Y5E-Tc; Thu, 02 Nov 2023 12:56:15 +0000 Date: Thu, 2 Nov 2023 12:56:15 +0000 From: Matthew Wilcox To: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Should vmap() work on pages or folios? Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Rspamd-Queue-Id: 2F6BA80011 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: bt6srox6ny37a83omc8rosmhcmea3mej X-HE-Tag: 1698929778-467464 X-HE-Meta: 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 RQ727Gi8 ONX/Pt2NdUxk7EQzWXpAsmp9hGhPYMc6ybTgpG5B1NWeHmTkhBWBHgVtkAF/V4eTv1uxybpk3iUS3ecMFCcF56A3kcLOweK0/2yYPmt6HJegeE5XKt2HsffMm0hPFhvh54g/OhEKtDHP6iiLgcRUu8ZlCmfBBxA3rK1Q/BTZMuGN7EqbTp9Pod8YgqQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000008, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: For various reasons, I started looking at converting vm_struct.pages to be vm_struct.folios. But vmap() has me wondering because it contains: if (flags & VM_MAP_PUT_PAGES) { area->pages = pages; area->nr_pages = count; } In principle, then, we could call vmap() with an array of pages that includes tail pages. However, I think if we do that today, things will go badly wrong. You see, despite the name of the flag, we don't actually call put_page(). Instead, we call __free_page() which calls __free_pages(page, 0), which calls put_page_testzero(). Since tail pages have a refcount of 0, it'll hit the VM_BUG_ON_PAGE(). >From this, I can conclude nobody does this today. But people might be calling vmap() with tail pages and VM_MAP_PUT_PAGES _not_ set. And it's not necessarily a stupid thing to want to stitch together some tail pages (from different folios) into a virtually contiguous block. I thibk the primary usecase is order-0 allocations being stuck together into a virtually contiguous block, but I haven't audited every caller of vmap. So what's our intent here? Should we fix vmap() to actually work with tail pages? Should we require vmap() to only work on order-0 pages?