Re: 3.15-rc8 oops in copy_page_rep after page fault.

[Kirill A. Shutemov]

On Sun, Jun 15, 2014 at 08:01:27PM -0700, Hugh Dickins wrote:
> On Fri, 6 Jun 2014, Sasha Levin wrote:
> > On 06/06/2014 02:49 PM, Kirill A. Shutemov wrote:
> > > On Fri, Jun 06, 2014 at 11:26:14AM -0700, Linus Torvalds wrote:
> > >> > On Fri, Jun 6, 2014 at 10:43 AM, Dave Jones <davej@xxxxxxxxxx> wrote:
> > >>> > >
> > >>> > > RIP: 0010:[<ffffffff8b3287b5>]  [<ffffffff8b3287b5>] copy_page_rep+0x5/0x10
> > >> > 
> > >> > Ok, it's the first iteration of "rep movsq" (%rcx is still 0x200) for
> > >> > copying a page, and the pages are
> > >> > 
> > >> >   RSI: ffff880052766000
> > >> >   RDI: ffff880014efe000
> > >> > 
> > >> > which both look like reasonable kernel addresses. So I'm assuming it's
> > >> > DEBUG_PAGEALLOC that makes this trigger, and since the error code is
> > >> > 0, and the CR2 value matches RSI, it's the source page that seems to
> > >> > have been freed.
> > >> > 
> > >> > And I see absolutely _zero_ reason for wht your 64k mmap_min_addr
> > >> > should make any difference what-so-ever. That's just odd.
> > >> > 
> > >> > Anyway, can you try to figure out _which_ copy_user_highpage() it is
> > >> > (by looking at what is around the call-site at
> > >> > "handle_mm_fault+0x1e0". The fact that we have a stale
> > >> > do_huge_pmd_wp_page() on the stack makes me suspect that we have hit
> > >> > that VM_FAULT_FALLBACK case and this is related to splitting. Adding a
> > >> > few more people explicitly to the cc in case anybody sees anything
> > >> > (original email on lkml and linux-mm for context, guys).
> > > Looks like a known false positive from DEBUG_PAGEALLOC:
> > > 
> > > https://lkml.org/lkml/2013/3/29/103
> > > 
> > > We huge copy page in do_huge_pmd_wp_page() without ptl taken and the page
> > > can be splitted and freed under us. Once page is copied we take ptl again
> > > and recheck that PMD is not changed. If changed, we don't use new page.
> > > Not a bug, never triggered with DEBUG_PAGEALLOC disabled.
> > > 
> > > It would be nice to have a way to mark this kind of speculative access.
> > 
> > FWIW, this issue makes fuzzing with DEBUG_PAGEALLOC nearly impossible since
> > this thing is so common we never get to do anything "fun" before this issue
> > triggers.
> > 
> > A fix would be more than welcome.
> 
> Please give this a try: I think it's right, but I could easily be wrong.
> 
> 
> [PATCH] thp: fix DEBUG_PAGEALLOC oops in copy_page_rep
> 
> Trinity has for over a year been reporting a CONFIG_DEBUG_PAGEALLOC
> oops in copy_page_rep() called from copy_user_huge_page() called from
> do_huge_pmd_wp_page().
> 
> I believe this is a DEBUG_PAGEALLOC false positive, due to the source
> page being split, and a tail page freed, while copy is in progress; and
> not a problem without DEBUG_PAGEALLOC, since the pmd_same() check will
> prevent a miscopy from being made visible.
> 
> Fix by adding get_user_huge_page() and put_user_huge_page(): reducing
> to the usual get_page() and put_page() on head page in the usual config;
> but get and put references to all of the tail pages when DEBUG_PAGEALLOC.
> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Ugly, but should do the job:

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

BTW, we will not need this with new THP refcounting I'm playing with:
reference on the page will be enough to protect against splitting.

-- 
 Kirill A. Shutemov
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