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]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id D8B2DCD8CA8 for ; Fri, 12 Jun 2026 03:45:08 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 257AF6B0005; Thu, 11 Jun 2026 23:45:08 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 206DB6B0088; Thu, 11 Jun 2026 23:45:08 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 11E2C6B008C; Thu, 11 Jun 2026 23:45:08 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 039E66B0005 for ; Thu, 11 Jun 2026 23:45:08 -0400 (EDT) Received: from smtpin01.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay07.hostedemail.com (Postfix) with ESMTP id C1757165A34 for ; Fri, 12 Jun 2026 03:45:07 +0000 (UTC) X-FDA: 84869869854.01.F4E7B9F Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf31.hostedemail.com (Postfix) with ESMTP id 7220F20003 for ; Fri, 12 Jun 2026 03:45:05 +0000 (UTC) Authentication-Results: imf31.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b="Omfx/M+u"; spf=pass (imf31.hostedemail.com: domain of willy@infradead.org designates 90.155.50.34 as permitted sender) smtp.mailfrom=willy@infradead.org; dmarc=pass (policy=none) header.from=infradead.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1781235906; h=from:from:sender: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:dkim-signature; bh=8wuNTTt8Nkmf44XvMl/+1kApdnUIs1lQ+xGFMDCtpqk=; b=Vdi5Ej7MIZq/dzaA9qgBsOEbjqJTqbT3HYBD2C7gSdkBtW9i4HHZqBD911ua6o3vSJjf/E nBHlq3aoIlYdt+dR7GJBvOuWJoYw0WAPX68c0fP24ks+tqA0zKIb29YWFl1ShorQ8aUWuK JUFPb60VYICQGUV2PtZ8zHMfmnBNE1g= ARC-Authentication-Results: i=1; imf31.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b="Omfx/M+u"; spf=pass (imf31.hostedemail.com: domain of willy@infradead.org designates 90.155.50.34 as permitted sender) smtp.mailfrom=willy@infradead.org; dmarc=pass (policy=none) header.from=infradead.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1781235906; b=5z/AVo30yLdDgmQmHSCut/RizRgVuurDrXVvCtalL9FhTi4aC0qM6+N7JKKx/nMlwQfiQy 3NxGLQiymJj25j2uw4AyOXMgkdEQbwsS6gFi5iR1Guvrr2fDsMHNnjlJM3VrWQZxA9OzGO zt/1GvKegVobadJApPNfXoYEuNOfGYs= DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=8wuNTTt8Nkmf44XvMl/+1kApdnUIs1lQ+xGFMDCtpqk=; b=Omfx/M+uj8H4iUlLYnVIlSoA0f 83mqDu1TOrrhlbKmtTjrtxG37j/CPmGVYlIdrf/ix6BxCHIi1LPlAFeg1e2YMZPBPRHUgf3S4LN5x PSGpxCML5T8HDnCKR0r6w/1fIUNw6HJzmnXr108sYmRS+Ll/s4fumypdB39yxuKiCUJDDVMet7fOz lkafYWbLoqw0lofci3jc7FN/eU9xasCTmwg1LLKB/UkkMA4F7dzYre2Z9xK9kLUClCmePnztc11Qg 0OGM9nbOeSZ8TM00KEsuAl8ZGFE5YJjtAVOl/DmoHZSSpYU1Fn1Urxg8nahiBFdfhW9BspxdlnoLe dH6wy6cw==; Received: from willy by casper.infradead.org with local (Exim 4.99.1 #2 (Red Hat Linux)) id 1wXsp5-000000033X5-0vpy; Fri, 12 Jun 2026 03:44:47 +0000 Date: Fri, 12 Jun 2026 04:44:47 +0100 From: Matthew Wilcox To: Zhaoyang Huang Cc: Zi Yan , jaegeuk@kernel.org, Chao Yu , "jyescas@google.com" , "David Hildenbrand (Arm)" , "zhaoyang.huang" , Andrew Morton , Lorenzo Stoakes , Barry Song , Baolin Wang , Lance Yang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain , linux-mm@kvack.org, linux-kernel@vger.kernel.org, steve.kang@unisoc.com, xiuhong.wang@unisoc.com, hao_hao.wang@unisoc.com Subject: Re: [RFC PATCH] mm/huge_memory: do not add dropped split tail folios to LRU Message-ID: References: <20260610120535.2370844-1-zhaoyang.huang@unisoc.com> <4348A64F-30F9-4497-A839-A80BCFFCFFE0@nvidia.com> <2F9A72CC-772C-4691-9F18-77306EADC6CB@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 7220F20003 X-Stat-Signature: wjjwggeqgcour6cw4o8nhydddrmotb61 X-Rspam-User: X-HE-Tag: 1781235905-251437 X-HE-Meta: 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 xELsw7Yq tW+Qg3c4LESR1ZUuyYXrTHMoIba/yVrX9qMefTy/tTtsj0KPoakqNSVUIY0qXckG4ejhLB8vjGfsrdvqOzScz2V7fCYq89Z0xS0Hai3uOQMZh4r0tl1OZ2IBmnhecjHMepVDOuWGAxi6i7/WpV/nh2qoW9tKvplf++wx3rtJJMMSfuHgUzgiskOLjTUy69HXN5beJWnuupoaf4xYhgnTQmyBir5S++QyEqZpSuIZTOd/8PWAdwjAZjl4mETCLPxyLFOic3aQ5zbUcjh4R0yAlRRJ8cFU+cf5VILxUgNrOkK4uQpoz/LJF2Gk65jB9vsLyVqJAj0nPVSnAa4avTZlVu7YGaGt0n5Xv26Wa3Utz2IeMED8Kdl8Nsp/JlULNnC4FnLBN Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jun 11, 2026 at 03:45:19PM +0800, Zhaoyang Huang wrote: > > > pin_user_pages*(..., FOLL_LONGTERM) > > > └─ __gup_longterm_locked() [gup.c:2465] > > > │ ├─ follow_page_pte() [gup.c:802] > > > │ │ └─ try_grab_folio() [gup.c:858] > > > if (WARN_ON_ONCE(folio_ref_count(folio) <= 0)) > > > return -ENOMEM; > > > > > > // Could __folio_split->folio_put could > > > race here ? > > > if (flags & FOLL_GET) > > > folio_ref_add(folio, refs); > > > └─ check_and_migrate_movable_pages() [gup.c:2490] > > > └─ collect_longterm_unpinnable_folios() [gup.c:2391] > > > └─ └─if (!folio_isolate_lru(folio)) > > > > > > Could the __folio_split race in the above scenario? It looks like > > > try_grab_folio set the refcount without using atomic operation. > > > > folio_ref_add() used by try_grab_folio() is an atomic op. > > Which refcount change is not atomic here? > The atomic I mean is folio_try_get is implemented by > atomic_add_unless, while try_grab_folio does this by the below > sequence which leaves a window to have __folio_split race with it. > right? > > if (WARN_ON_ONCE(folio_ref_count(folio) <= 0)) > .... > > if (flags & FOLL_GET) > folio_ref_add(folio, refs); No. try_grab_folio() is only called when we already have a reference to the folio. It says so right in the comment: * It is called when we have a stable reference for the folio, typically in * GUP slow path. You may not be able to see it; in the GUP slow path, we have a lock preventing unmapping, and the folio is mapped, therefore there's a reference count on the folio already. This isn't true for try_grab_folio_fast(), which is why it uses try_get instead of ref_add.