Messages in this thread | | | Date | Wed, 1 Aug 2012 11:05:28 -0400 | From | Konrad Rzeszutek Wilk <> | Subject | Re: [stable] backport "xen: mark local pages as FOREIGN in the m2p_override" |
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On Wed, Aug 01, 2012 at 02:34:08PM +0100, Stefano Stabellini wrote: > Hello, > I would like to request a backport of the following upstream Linux > commit to 3.4, 3.3, 3.2, 3.1, 3.0, 2.6.39 and 2.6.38. > It fixes a deadlock that happens when a Xen frontend driver connects to > a Xen backend driver in the same domain. A detailed explanation is > included in the commit message. > > A simple cherry-pick should work for all the stable versions.
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Thank you! > > Thanks, > > Stefano > > > commit b9e0d95c041ca2d7ad297ee37c2e9cfab67a188f > Author: Stefano Stabellini <stefano.stabellini@eu.citrix.com> > Date: Wed May 23 18:57:20 2012 +0100 > > xen: mark local pages as FOREIGN in the m2p_override > > When the frontend and the backend reside on the same domain, even if we > add pages to the m2p_override, these pages will never be returned by > mfn_to_pfn because the check "get_phys_to_machine(pfn) != mfn" will > always fail, so the pfn of the frontend will be returned instead > (resulting in a deadlock because the frontend pages are already locked). > > INFO: task qemu-system-i38:1085 blocked for more than 120 seconds. > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > qemu-system-i38 D ffff8800cfc137c0 0 1085 1 0x00000000 > ffff8800c47ed898 0000000000000282 ffff8800be4596b0 00000000000137c0 > ffff8800c47edfd8 ffff8800c47ec010 00000000000137c0 00000000000137c0 > ffff8800c47edfd8 00000000000137c0 ffffffff82213020 ffff8800be4596b0 > Call Trace: > [<ffffffff81101ee0>] ? __lock_page+0x70/0x70 > [<ffffffff81a0fdd9>] schedule+0x29/0x70 > [<ffffffff81a0fe80>] io_schedule+0x60/0x80 > [<ffffffff81101eee>] sleep_on_page+0xe/0x20 > [<ffffffff81a0e1ca>] __wait_on_bit_lock+0x5a/0xc0 > [<ffffffff81101ed7>] __lock_page+0x67/0x70 > [<ffffffff8106f750>] ? autoremove_wake_function+0x40/0x40 > [<ffffffff811867e6>] ? bio_add_page+0x36/0x40 > [<ffffffff8110b692>] set_page_dirty_lock+0x52/0x60 > [<ffffffff81186021>] bio_set_pages_dirty+0x51/0x70 > [<ffffffff8118c6b4>] do_blockdev_direct_IO+0xb24/0xeb0 > [<ffffffff811e71a0>] ? ext3_get_blocks_handle+0xe00/0xe00 > [<ffffffff8118ca95>] __blockdev_direct_IO+0x55/0x60 > [<ffffffff811e71a0>] ? ext3_get_blocks_handle+0xe00/0xe00 > [<ffffffff811e91c8>] ext3_direct_IO+0xf8/0x390 > [<ffffffff811e71a0>] ? ext3_get_blocks_handle+0xe00/0xe00 > [<ffffffff81004b60>] ? xen_mc_flush+0xb0/0x1b0 > [<ffffffff81104027>] generic_file_aio_read+0x737/0x780 > [<ffffffff813bedeb>] ? gnttab_map_refs+0x15b/0x1e0 > [<ffffffff811038f0>] ? find_get_pages+0x150/0x150 > [<ffffffff8119736c>] aio_rw_vect_retry+0x7c/0x1d0 > [<ffffffff811972f0>] ? lookup_ioctx+0x90/0x90 > [<ffffffff81198856>] aio_run_iocb+0x66/0x1a0 > [<ffffffff811998b8>] do_io_submit+0x708/0xb90 > [<ffffffff81199d50>] sys_io_submit+0x10/0x20 > [<ffffffff81a18d69>] system_call_fastpath+0x16/0x1b > > The explanation is in the comment within the code: > > We need to do this because the pages shared by the frontend > (xen-blkfront) can be already locked (lock_page, called by > do_read_cache_page); when the userspace backend tries to use them > with direct_IO, mfn_to_pfn returns the pfn of the frontend, so > do_blockdev_direct_IO is going to try to lock the same pages > again resulting in a deadlock. > > A simplified call graph looks like this: > > pygrub QEMU > ----------------------------------------------- > do_read_cache_page io_submit > | | > lock_page ext3_direct_IO > | > bio_add_page > | > lock_page > > Internally the xen-blkback uses m2p_add_override to swizzle (temporarily) > a 'struct page' to have a different MFN (so that it can point to another > guest). It also can easily find out whether another pfn corresponding > to the mfn exists in the m2p, and can set the FOREIGN bit > in the p2m, making sure that mfn_to_pfn returns the pfn of the backend. > > This allows the backend to perform direct_IO on these pages, but as a > side effect prevents the frontend from using get_user_pages_fast on > them while they are being shared with the backend. > > Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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