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


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