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[184.147.118.108]) by smtp.gmail.com with ESMTPSA id s5sm2378221qtn.33.2021.06.03.14.48.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Jun 2021 14:48:48 -0700 (PDT) Date: Thu, 3 Jun 2021 17:48:46 -0400 From: Peter Xu To: Hugh Dickins Cc: 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 Message-ID: References: MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Queue-Id: AE16A42110AD Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=iRPuDSHt; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf10.hostedemail.com: domain of peterx@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=peterx@redhat.com X-Rspamd-Server: rspam03 X-Stat-Signature: sukw9gapf77fsdwwhpb6fbpx74j9qejk X-HE-Tag: 1622756945-900296 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 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().. And.. isn't PVMW_SYNC only meaningful for pte-level only (as I didn't see a reference of it outside map_pte)? -- Peter Xu