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

[David Hildenbrand]

On 26.10.22 23:51, 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).
>
> But my eye on COW is very poor nowadays, and I may be plain wrong.

I am not aware of any reason for copying a !anon page during fork(). COW 
regrading fork() is all about sharing private (anon) pages between the 
parent and the child. The semantics of other pages are untouched.


Yes, I am working on reliable longterm R/O pinning improvements, whereby 
we never pin such pages in a MAP_PRIVATE mapping but instead break COW 
before pinning; but this only applies to longterm pinning 
(FOLL_LONGTERM) and is independent of fork() here.


Let me elaborate: if you have a pagecache page (or the shared zeropage) 
in a MAP_PRIVATE mapping pinned R/O, the next write fault will replace 
the page by a copy, *independent* of fork() or not: the page is already 
mapped write-protected into the page table.


IIUC, the problem here is that we have a writable private mapping (COW 
mapping) of, say, a file, whereby we never had to COW -- so no anon 
pages were mapped.

Then, we had the process pin some page (&src_mm->has_pinned) once and 
detect a pagecache page / shared zeropage as "maybe pinned" during 
fork(), which can happen easily, for example, due to other process' 
action, false positives, ... we end up duplicating a !anon page.

Restricting copying during fork() to anon pages is IMHO the right thing 
to do.

--
Thanks,

David / dhildenb