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Re: [Xen-devel] [PATCH] xen/blkback: Avoid unmapping unmapped grant pages

On 11/26/19 8:36 AM, SeongJae Park wrote:
From: SeongJae Park <sjpark@xxxxxxxxx>

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

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 
   [  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 
   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")

Queued up with Roger's reviewed-by.

Jens Axboe

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