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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id x18sm3110686qkx.118.2021.06.03.19.54.12 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Thu, 03 Jun 2021 19:54:13 -0700 (PDT) Date: Thu, 3 Jun 2021 19:54:11 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Peter Xu cc: Hugh Dickins , Andrew Morton , "Kirill A. Shutemov" , Yang Shi , Wang Yugui , Matthew Wilcox , Naoya Horiguchi , Alistair Popple , Ralph Campbell , Zi Yan , Miaohe Lin , Minchan Kim , Jue Wang , Jan Kara , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/7] mm/thp: try_to_unmap() use TTU_SYNC for safe DEBUG_VM splitting In-Reply-To: Message-ID: References: User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Stat-Signature: wy7oy3cndnj3zgztymextpexu3nwoigf X-Rspamd-Queue-Id: 2613F4229B8D X-Rspamd-Server: rspam02 Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=google.com header.s=20161025 header.b=Ti9OjQCN; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf02.hostedemail.com: domain of hughd@google.com designates 209.85.219.44 as permitted sender) smtp.mailfrom=hughd@google.com X-HE-Tag: 1622775251-569079 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 Thu, 3 Jun 2021, Peter Xu wrote: > On Tue, Jun 01, 2021 at 02:07:53PM -0700, Hugh Dickins wrote: > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > > index 2cf01d933f13..b45d22738b45 100644 > > --- a/mm/page_vma_mapped.c > > +++ b/mm/page_vma_mapped.c > > @@ -212,6 +212,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > > pvmw->ptl = NULL; > > } > > } else if (!pmd_present(pmde)) { > > + /* > > + * If PVMW_SYNC, take and drop THP pmd lock so that we > > + * cannot return prematurely, while zap_huge_pmd() has > > + * cleared *pmd but not decremented compound_mapcount(). > > + */ > > + if ((pvmw->flags & PVMW_SYNC) && > > + PageTransCompound(pvmw->page)) > > + spin_unlock(pmd_lock(mm, pvmw->pmd)); > > return false; > > } > > if (!map_pte(pvmw)) > > Sorry if I missed something important, but I'm totally confused on how this > unlock is pairing with another lock().. I imagine you're reading that as spin_unlock(pmd_lockptr(blah)); no, the lock is right there, inside spin_unlock(pmd_lock(blah)). > > And.. isn't PVMW_SYNC only meaningful for pte-level only (as I didn't see a > reference of it outside map_pte)? But you are pointing directly to its reference outside map_pte()! Hugh