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[70.24.86.62]) by smtp.gmail.com with ESMTPSA id t7-20020a05622a01c700b003f6ac526568sm3112027qtw.39.2023.05.24.15.31.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 May 2023 15:31:20 -0700 (PDT) Date: Wed, 24 May 2023 18:31:18 -0400 From: Peter Xu To: Hugh Dickins Cc: Andrew Morton , Mike Kravetz , Mike Rapoport , "Kirill A. Shutemov" , Matthew Wilcox , David Hildenbrand , Suren Baghdasaryan , Qi Zheng , Yang Shi , Mel Gorman , Peter Zijlstra , Will Deacon , Yu Zhao , Alistair Popple , Ralph Campbell , Ira Weiny , Steven Price , SeongJae Park , Naoya Horiguchi , Christophe Leroy , Zack Rusin , Jason Gunthorpe , Axel Rasmussen , Anshuman Khandual , Pasha Tatashin , Miaohe Lin , Minchan Kim , Christoph Hellwig , Song Liu , Thomas Hellstrom , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 14/31] fs/userfaultfd: retry if pte_offset_map() fails Message-ID: References: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com> <424274a4-7c13-e14-b380-428fc69a45c5@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <424274a4-7c13-e14-b380-428fc69a45c5@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, May 21, 2023 at 10:06:32PM -0700, Hugh Dickins wrote: > Instead of worrying whether the pmd is stable, userfaultfd_must_wait() > call pte_offset_map() as before, but go back to try again if that fails. > > Risk of endless loop? It already broke out if pmd_none(), !pmd_present() > or pmd_trans_huge(), and pte_offset_map() would have cleared pmd_bad(): > which leaves pmd_devmap(). Presumably pmd_devmap() is inappropriate in > a vma subject to userfaultfd (it would have been mistreated before), > but add a check just to avoid all possibility of endless loop there. Agreed, afaiu that's for either PFNMAP or MIXEDMAP vmas only. Maybe we can use a WARN_ON_ONCE() for that to be clear, but no strong opinions. > > Signed-off-by: Hugh Dickins Acked-by: Peter Xu -- Peter Xu