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

[<sjpark@amazon.com>]

On 27.11.19 10:13, Roger Pau Monné wrote:
> 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.


May I ask some more comments?



Thanks,

SeongJae Park