3.16.82-rc1 review patch. If anyone has any objections, please let me know.
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From: SeongJae Park sjpark@amazon.de
commit f9bd84a8a845d82f9b5a081a7ae68c98a11d2e84 upstream.
For each I/O request, blkback first maps the foreign pages for the request to its local pages. If an allocation of a local page for the mapping fails, it should unmap every mapping already made for the request.
However, blkback's handling mechanism for the allocation failure does not mark the remaining foreign pages as unmapped. Therefore, the unmap function merely tries to unmap every valid grant page for the request, including the pages not mapped due to the allocation failure. On a system that fails the allocation frequently, this problem leads to following kernel crash.
[ 372.012538] BUG: unable to handle kernel NULL pointer dereference at 0000000000000001 [ 372.012546] IP: [<ffffffff814071ac>] gnttab_unmap_refs.part.7+0x1c/0x40 [ 372.012557] PGD 16f3e9067 PUD 16426e067 PMD 0 [ 372.012562] Oops: 0002 [#1] SMP [ 372.012566] Modules linked in: act_police sch_ingress cls_u32 ... [ 372.012746] Call Trace: [ 372.012752] [<ffffffff81407204>] gnttab_unmap_refs+0x34/0x40 [ 372.012759] [<ffffffffa0335ae3>] xen_blkbk_unmap+0x83/0x150 [xen_blkback] ... [ 372.012802] [<ffffffffa0336c50>] dispatch_rw_block_io+0x970/0x980 [xen_blkback] ... Decompressing Linux... Parsing ELF... done. Booting the kernel. [ 0.000000] Initializing cgroup subsys cpuset
This commit fixes this problem by marking the grant pages of the given request that didn't mapped due to the allocation failure as invalid.
Fixes: c6cc142dac52 ("xen-blkback: use balloon pages for all mappings")
Reviewed-by: David Woodhouse dwmw@amazon.de Reviewed-by: Maximilian Heyne mheyne@amazon.de Reviewed-by: Paul Durrant pdurrant@amazon.co.uk Reviewed-by: Roger Pau Monné roger.pau@citrix.com Signed-off-by: SeongJae Park sjpark@amazon.de Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Ben Hutchings ben@decadent.org.uk --- drivers/block/xen-blkback/blkback.c | 2 ++ 1 file changed, 2 insertions(+)
--- a/drivers/block/xen-blkback/blkback.c +++ b/drivers/block/xen-blkback/blkback.c @@ -825,6 +825,8 @@ next: out_of_memory: pr_alert(DRV_PFX "%s: out of memory\n", __func__); put_free_pages(blkif, pages_to_gnt, segs_to_map); + for (i = last_map; i < num; i++) + pages[i]->handle = BLKBACK_INVALID_HANDLE; return -ENOMEM; }