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SubjectRe: [PATCH 10 of 20] ipath - support for userspace apps using core driver
On Wed, 15 Mar 2006, Andrew Morton wrote:
> "Bryan O'Sullivan" <> wrote:
> >
> > Bad page state at __free_pages_ok (in process 'mpi_hello', page ffff8100020e2f88)
> > flags:0x0100000000000804 mapping:0000000000000000 mapcount:0 count:0 (Not tainted)
> Someone left PG_private set on this page (!)

That's pretty sure to be from page_alloc.c's internal use of
PagePrivate, for the buddy system of free pages.

I think Bryan's problem is probably that he has allocated a
high-order page (via pci_alloc_consistent or dma_alloc_coherent),
and is then exposing the constituents of that high-order page to
userspace via nopage: when the final munmap or exit comes,
the constituent pages get freed without realizing they're
supposed to be part of one high-order page.

We had the same problem in sound/core/memalloc.c, and fixed it
by using a compound page (a high-order page in which each constituent
knows it's part of the larger whole): see the two uses of __GFP_COMP
in that file, and in particular the one where snd_malloc_dev_pages
calls dma_alloc_coherent.

Yes, as Roland suggested, it sounds sensible to use dma_alloc_coherent
throughout rather than pci_alloc_consistent, then you can pass __GFP_COMP
down with the other flags and it should work (without any PageReserved).

I haven't checked Bryan's get_pag'ing, but he seemed to have the right
idea of how it should work: was just being a bit diffident about it in
the face of the fact that it clearly wasn't working (through lack of

Of course, there is an alternative way of going about it, using
remap_pfn_range more (VM_PFNMAP keeping the VM away from refcounting
the pages) and nopage less. But I think Bryan should probably keep on
with the division he already has, no strong reason to change it around.

[ snipping lots of helpful explanation and advice from Andrew ]

> If you have pages which are created by ->nopage() but which are supposed to
> be shared across forks (the VMA should use VM_DONTCOPY|VM_SHARED) then each
> time a new process starts accessing these already-existing pages it'll take
> a minor fault.

No. That isn't how VM_DONTCOPY behaves, and I don't know what you were
thinking to bring it into the discussion there - if you have pages which
are supposed to be shared across forks, that happens without any special
flag, doesn't it? We only teach VM_DONTCOPY in the second year,
I think Bryan would best forget it for now ;)

Oh well, I'd better say a bit on it. VM_DONTCOPY excludes the whole vma
from being forked into the child. So if the child tries to access one
of those addresses, the child'll get a SIGSEGV not a minor fault. RDMA
is finding it a useful flag to get around a peculiarly frustrating issue
with get_user_pages: though that successfully pins pages, a fork followed
by parent writing to pinned private page will put a _copy_ of that page
into the parent's userspace (if child holds a reference to the original),
which likely defeats the purpose of pinning.

So far as Bryan is dealing with shared mappings, he won't be interested
in VM_DONTCOPY at all. If he's dealing with private mappings, he might
want to consider VM_DONTCOPY if get_user_pages and fork can occur
concurrently; but it's an advanced topic, right now he wants to get
the basis working without "Bad page state"s.

He shouldn't set VM_SHM, it means nothing. If he's dealing with real
allocated pages, he shouldn't set VM_IO (but remap_pfn_range will set
it for him if it's used). He shouldn't set VM_RESERVED (won't do any
harm, but we're quite likely to remove it soon). He can set VM_DONT-
EXPAND to avoid worrying about whether mremap to a larger extent,
followed by fault on that larger extent, would behave correctly.
He shouldn't set VM_LOCKED (it would make the locked_vm accounting
go wrong - unless he's careful to participate in that accounting,
as Infiniband's uverbs_mem.c creditably does).

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