Re: [Xen-devel] spurious warnings from get_page() via gnttab_copy() during frontend shutdown

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

I don't think you should get a backtrace. Did you add that?

 -- Keir

On 27/11/07 09:26, "David Edmondson" <dme@xxxxxxx> wrote:

> In testing our implementation of the hypervisor copy based backend-
>> frontend networking changes, we see what I believe are spurious
> warning messages during the shutdown of the frontend domain:
> 
> (XEN) /export/build/schuster/xvm-gate/xen.hg/xen/include/asm/mm.h:
> 189:d0 Error pfn 30e290: rd=ffff830000fcf100, od=ffff830000fcf100,
> caf=00000000, taf=0000000000000000
> (XEN) Xen call trace:
> (XEN)    [<ffff83000010f240>] get_page+0x107/0x1b4
> (XEN)    [<ffff83000010f10a>] get_page_and_type+0x21/0x50
> (XEN)    [<ffff8300001116c4>] __gnttab_copy+0x3f5/0x5b4
> (XEN)    [<ffff830000111971>] gnttab_copy+0xee/0x1c4
> (XEN)    [<ffff830000111dbd>] do_grant_table_op+0x376/0x3bc
> (XEN)    [<ffff8300001b83e2>] syscall_enter+0xa2/0xfc
> (XEN)
> (XEN) Guest stack trace from rbp=ffff830000ff3cf8:
> (XEN)  ???????????????? <G><2>grant_table.c:990:d0 do_grant_table_op:
> domain 0, cmd 5, count 1
> (XEN) /export/build/schuster/xvm-gate/xen.hg/xen/include/asm/mm.h:
> 189:d0 Error pfn 30fc2a: rd=ffff830000fcf100, od=0000000000000000,
> caf=00000000, taf=0000000000000000
> (XEN) Xen call trace:
> (XEN)    [<ffff83000010f240>] get_page+0x107/0x1b4
> (XEN)    [<ffff83000010f10a>] get_page_and_type+0x21/0x50
> (XEN)    [<ffff8300001116c4>] __gnttab_copy+0x3f5/0x5b4
> (XEN)    [<ffff830000111971>] gnttab_copy+0xee/0x1c4
> (XEN)    [<ffff830000111dbd>] do_grant_table_op+0x376/0x3bc
> (XEN)    [<ffff8300001b83e2>] syscall_enter+0xa2/0xfc
> (XEN)
> 
> What we think is happening is that the frontend dies and most of its'
> pages are freed (those that are not referenced by another domain).
> The backend doesn't know that the frontend died yet, so it's still
> trying to pass packets along to it. It has the rx ring mapped
> (meaning that it can't be freed) and reads previously advertised
> grant references from it. Those grants now refer to pages that are no
> longer valid, so get_page() complains (the pages are no longer valid
> as only the frontend had references to them and they were freed).
> 
> __gnttab_copy() itself seems prepared for this situation, as failures
> to grab the target page due to a dying domain are correctly handled:
> 
>      if ( !get_page_and_type(mfn_to_page(d_frame), dd,
> PGT_writable_page) )
>      {
>          if ( !test_bit(_DOMF_dying, &dd->domain_flags) )
>              gdprintk(XENLOG_WARNING, "Could not get dst frame %lx
> \n", d_frame);
>          rc = GNTST_general_error;
>          goto error_out;
>      }
> 
> In our testing we believe that we're following this path (_DOMF_dying
> is set and rc == GNTST_general_error) and that we handle the failure
> correctly.
> 
> The corresponding failure mode in the page flip code path doesn't
> result in any INFO warnings. Should they exist in this case?
> 
> dme.



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