linux-kernel - Re: [PATCH] cma: don't quit at first error when activating reserved areas

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Message-ID: <20200730191618.GA703407@carbon.DHCP.thefacebook.com>
Date:   Thu, 30 Jul 2020 12:16:18 -0700
From:   Roman Gushchin <guro@...com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Michal Nazarewicz <mina86@...a86.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH] cma: don't quit at first error when activating reserved
 areas

On Thu, Jul 30, 2020 at 09:31:23AM -0700, Mike Kravetz wrote:
> The routine cma_init_reserved_areas is designed to activate all
> reserved cma areas.  It quits when it first encounters an error.
> This can leave some areas in a state where they are reserved but
> not activated.  There is no feedback to code which performed the
> reservation.  Attempting to allocate memory from areas in such a
> state will result in a BUG.
> 
> Modify cma_init_reserved_areas to always attempt to activate all
> areas.  The called routine, cma_activate_area is responsible for
> leaving the area in a valid state.  No one is making active use
> of returned error codes, so change the routine to void.
> 
> How to reproduce:  This example uses kernelcore, hugetlb and cma
> as an easy way to reproduce.  However, this is a more general cma
> issue.
> 
> Two node x86 VM 16GB total, 8GB per node
> Kernel command line parameters, kernelcore=4G hugetlb_cma=8G
> Related boot time messages,
>   hugetlb_cma: reserve 8192 MiB, up to 4096 MiB per node
>   cma: Reserved 4096 MiB at 0x0000000100000000
>   hugetlb_cma: reserved 4096 MiB on node 0
>   cma: Reserved 4096 MiB at 0x0000000300000000
>   hugetlb_cma: reserved 4096 MiB on node 1
>   cma: CMA area hugetlb could not be activated
> 
>  # echo 8 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> 
>   BUG: kernel NULL pointer dereference, address: 0000000000000000
>   #PF: supervisor read access in kernel mode
>   #PF: error_code(0x0000) - not-present page
>   PGD 0 P4D 0
>   Oops: 0000 [#1] SMP PTI
>   ...
>   Call Trace:
>     bitmap_find_next_zero_area_off+0x51/0x90
>     cma_alloc+0x1a5/0x310
>     alloc_fresh_huge_page+0x78/0x1a0
>     alloc_pool_huge_page+0x6f/0xf0
>     set_max_huge_pages+0x10c/0x250
>     nr_hugepages_store_common+0x92/0x120
>     ? __kmalloc+0x171/0x270
>     kernfs_fop_write+0xc1/0x1a0
>     vfs_write+0xc7/0x1f0
>     ksys_write+0x5f/0xe0
>     do_syscall_64+0x4d/0x90
>     entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Fixes: c64be2bb1c6e ("drivers: add Contiguous Memory Allocator")
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: <stable@...r.kernel.org>

Makes total sense to me!

Reviewed-by: Roman Gushchin <guro@...com>

Thanks!