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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Nov 26, 2019 at 04:36:05PM +0100, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> 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")
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> 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>

Thanks, Roger.