Re: [PATCH STABLE 5.10] mm/memory: add non-anonymous page check in the copy_present_page()

[Peter Xu]

Hi, Yuanzheng,

On Wed, Oct 26, 2022 at 02:51:16PM -0700, Hugh Dickins wrote:
> On Mon, 24 Oct 2022, Yuanzheng Song wrote:
> 
> > The vma->anon_vma of the child process may be NULL because
> > the entire vma does not contain anonymous pages. In this
> > case, a BUG will occur when the copy_present_page() passes
> > a copy of a non-anonymous page of that vma to the
> > page_add_new_anon_rmap() to set up new anonymous rmap.
> > 
> > ------------[ cut here ]------------
> > kernel BUG at mm/rmap.c:1044!
> > Internal error: Oops - BUG: 0 [#1] SMP
> > Modules linked in:
> > CPU: 2 PID: 3617 Comm: test Not tainted 5.10.149 #1
> > Hardware name: linux,dummy-virt (DT)
> > pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--)
> > pc : __page_set_anon_rmap+0xbc/0xf8
> > lr : __page_set_anon_rmap+0xbc/0xf8
> > sp : ffff800014c1b870
> > x29: ffff800014c1b870 x28: 0000000000000001
> > x27: 0000000010100073 x26: ffff1d65c517baa8
> > x25: ffff1d65cab0f000 x24: ffff1d65c416d800
> > x23: ffff1d65cab5f248 x22: 0000000020000000
> > x21: 0000000000000001 x20: 0000000000000000
> > x19: fffffe75970023c0 x18: 0000000000000000
> > x17: 0000000000000000 x16: 0000000000000000
> > x15: 0000000000000000 x14: 0000000000000000
> > x13: 0000000000000000 x12: 0000000000000000
> > x11: 0000000000000000 x10: 0000000000000000
> > x9 : ffffc3096d5fb858 x8 : 0000000000000000
> > x7 : 0000000000000011 x6 : ffff5a5c9089c000
> > x5 : 0000000000020000 x4 : ffff5a5c9089c000
> > x3 : ffffc3096d200000 x2 : ffffc3096e8d0000
> > x1 : ffff1d65ca3da740 x0 : 0000000000000000
> > Call trace:
> >  __page_set_anon_rmap+0xbc/0xf8
> >  page_add_new_anon_rmap+0x1e0/0x390
> >  copy_pte_range+0xd00/0x1248
> >  copy_page_range+0x39c/0x620
> >  dup_mmap+0x2e0/0x5a8
> >  dup_mm+0x78/0x140
> >  copy_process+0x918/0x1a20
> >  kernel_clone+0xac/0x638
> >  __do_sys_clone+0x78/0xb0
> >  __arm64_sys_clone+0x30/0x40
> >  el0_svc_common.constprop.0+0xb0/0x308
> >  do_el0_svc+0x48/0xb8
> >  el0_svc+0x24/0x38
> >  el0_sync_handler+0x160/0x168
> >  el0_sync+0x180/0x1c0
> > Code: 97f8ff85 f9400294 17ffffeb 97f8ff82 (d4210000)
> > ---[ end trace a972347688dc9bd4 ]---
> > Kernel panic - not syncing: Oops - BUG: Fatal exception
> > SMP: stopping secondary CPUs
> > Kernel Offset: 0x43095d200000 from 0xffff800010000000
> > PHYS_OFFSET: 0xffffe29a80000000
> > CPU features: 0x08200022,61806082
> > Memory Limit: none
> > ---[ end Kernel panic - not syncing: Oops - BUG: Fatal exception ]---
> > 
> > This problem has been fixed by the fb3d824d1a46
> > ("mm/rmap: split page_dup_rmap() into page_dup_file_rmap() and page_try_dup_anon_rmap()"),
> > but still exists in the linux-5.10.y branch.
> > 
> > This patch is not applicable to this version because
> > of the large version differences. Therefore, fix it by
> > adding non-anonymous page check in the copy_present_page().
> > 
> > Fixes: 70e806e4e645 ("mm: Do early cow for pinned pages during fork() for ptes")
> > Signed-off-by: Yuanzheng Song <songyuanzheng@xxxxxxxxxx>
> 
> It's a good point, but this patch should not go into any stable release
> without an explicit Ack from either Peter Xu or David Hildenbrand.
> 
> To my eye, it's simply avoiding the issue, rather than fixing
> it properly; and even if the issue is so rare, and fixing properly
> too difficult or inefficent (a cached anon_vma?), that a workaround
> is good enough, it still looks like the wrong workaround (checking
> dst_vma->anon_vma instead of PageAnon seems more to the point, and
> less lenient).

Sorry to have missed this patch. I agree with Hugh that this patch may not
really fix the issue.

IIUC it's the case where the vma is privately mapping a file.  Some shared
pages got pinned, and here we're trying to trigger the CoW assuming it's
anonymous page but it's not.

The pin should be RO - if it was a write pin, CoW should have happened on
the page cache and there should be an anonymous page, and anon_vma should
be there, no issue should happen.  Only if with RO pin, we won't trigger
CoW, we won't have any anonymous page, we won't have anon_vma, hence the
panic.

The thing is if the page is RO pinned, skip copying it (as what was done in
this patch) is not correct either, because e.g. a follow up write after
fork() from the parent will trigger CoW and the dma RO page that was pinned
will be inconsistent to the page in pgtable anymore, I think.

IIUC the correct fix is what David worked on with unshare - when RO pin the
page cache we should have triggered CoR already before fork().  But as you
mentioned, that's too much change for stable.

So besides this workaround which seems feasible to at least not panic the
system (Hugh: I can't quickly tell what'll be the difference here to check
dst anon_vma or PageAnon, they all seem to work?  I could have missed
something for sure..), the other workaround is teach the user app to switch
to use RW pin for any DMA pages even if RO, so that it'll always guarantee
page coherence even with the old kernel, and afaict that's what RDMA relied
on for years.  I don't know an easy way to make old kernel work with RO pin
solidly if without the unsharing logic.

Thanks,

-- 
Peter Xu