linux-kernel - Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

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Date:   Tue, 29 Dec 2020 01:12:37 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jan Kara <jack@...e.cz>, Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries
 when prefaulting

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 <kirill@...temov.name> 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