Messages in this thread | | | Date | Thu, 04 Jul 2013 13:57:16 +0100 | From | Alex Bligh <> | Subject | Re: kernel panic in skb_copy_bits |
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--On 4 July 2013 03:12:10 -0700 Eric Dumazet <eric.dumazet@gmail.com> wrote:
> It looks like a typical COW issue to me. > > If the page content is written while there is still a reference on this > page, we should allocate a new page and copy the previous content. > > And this has little to do with networking.
I suspect this would get more attention if we could make Ian's case below trigger (a) outside Xen, (b) outside networking.
> memset(buf, 0xaa, 4096); > write(fd, buf, 4096) > memset(buf, 0x55, 4096); > (where fd is O_DIRECT on NFS) Can result in 0x55 being seen on the wire > in the TCP retransmit.
We know this should fail using O_DIRECT+NFS. We've had reports suggesting it fails in O_DIRECT+iSCSI. However, that's been with a kernel panic (under Xen) rather than data corruption as per the above.
Historical trawling suggests this is an issue with DRDB (see Ian's original thread from the mists of time).
I don't quite understand why we aren't seeing corruption with standard ATA devices + O_DIRECT and no Xen involved at all.
My memory is a bit misty on this but I had thought the reason why this would NOT be solved simply by O_DIRECT taking a reference to the page was that the O_DIRECT I/O completed (and thus the reference would be freed up) before the networking stack had actually finished with the page. If the O_DIRECT I/O did not complete until the page was actually finished with, we wouldn't see the problem in the first place. I may be completely off base here.
-- Alex Bligh
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