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Shutemov" To: Linus Torvalds Cc: Hugh Dickins , Matthew Wilcox , "Kirill A. Shutemov" , Will Deacon , Linux Kernel Mailing List , Linux-MM , Linux ARM , Catalin Marinas , Jan Kara , Minchan Kim , Andrew Morton , Vinayak Menon , Android Kernel Team Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting Message-ID: <20201228221237.6nu75kgxq7ikxn2a@box> References: <20201226224016.dxjmordcfj75xgte@box> <20201227234853.5mjyxcybucts3kbq@box> <20201228125352.phnj2x2ci3kwfld5@box> <20201228220548.57hl32mmrvvefj6q@box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201228220548.57hl32mmrvvefj6q@box> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 29, 2020 at 01:05:48AM +0300, Kirill A. Shutemov wrote: > On Mon, Dec 28, 2020 at 10:47:36AM -0800, Linus Torvalds wrote: > > On Mon, Dec 28, 2020 at 4:53 AM Kirill A. Shutemov wrote: > > > > > > So far I only found one more pin leak and always-true check. I don't see > > > how can it lead to crash or corruption. Keep looking. > > > > Well, I noticed that the nommu.c version of filemap_map_pages() needs > > fixing, but that's obviously not the case Hugh sees. > > > > No,m I think the problem is the > > > > pte_unmap_unlock(vmf->pte, vmf->ptl); > > > > at the end of filemap_map_pages(). > > > > Why? > > > > Because we've been updating vmf->pte as we go along: > > > > vmf->pte += xas.xa_index - last_pgoff; > > > > and I think that by the time we get to that "pte_unmap_unlock()", > > vmf->pte potentially points to past the edge of the page directory. > > Well, if it's true we have bigger problem: we set up an pte entry without > relevant PTL. > > But I *think* we should be fine here: do_fault_around() limits start_pgoff > and end_pgoff to stay within the page table. > > It made mw looking at the code around pte_unmap_unlock() and I think that > the bug is that we have to reset vmf->address and NULLify vmf->pte once we > are done with faultaround: > > diff --git a/mm/memory.c b/mm/memory.c Ugh.. Wrong place. Need to sleep. I'll look into your idea tomorrow. diff --git a/mm/filemap.c b/mm/filemap.c index 87671284de62..e4daab80ed81 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2987,6 +2987,8 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf, unsigned long address, } while ((head = next_map_page(vmf, &xas, end_pgoff)) != NULL); pte_unmap_unlock(vmf->pte, vmf->ptl); rcu_read_unlock(); + vmf->address = address; + vmf->pte = NULL; WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss); return ret; -- Kirill A. Shutemov