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Message-ID: <20191127091314.GK980@Air-de-Roger>
Date: Wed, 27 Nov 2019 10:13:14 +0100
From: Roger Pau Monné <roger.pau@...rix.com>
To: SeongJae Park <sjpark@...zon.com>
CC: <konrad.wilk@...cle.com>, <axboe@...nel.dk>,
<xen-devel@...ts.xenproject.org>, <linux-block@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, SeongJae Park <sjpark@...zon.de>
Subject: Re: [PATCH] xen/blkback: Avoid unmapping unmapped grant pages
On Tue, Nov 26, 2019 at 04:36:05PM +0100, SeongJae Park wrote:
> From: SeongJae Park <sjpark@...zon.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@...zon.de>
> Reviewed-by: David Woodhouse <dwmw@...zon.de>
> Reviewed-by: Maximilian Heyne <mheyne@...zon.de>
> Reviewed-by: Paul Durrant <pdurrant@...zon.co.uk>
Reviewed-by: Roger Pau Monné <roger.pau@...rix.com>
Thanks, Roger.
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