Re: [PATCH 2/2] memory-hotplug: don't replace lowmem pages with highmem

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 12 Sep 2012 10:43:51 +0900
Minchan Kim <minchan@kernel.org> wrote:

> [1] reporeted that lowmem pages could be replaced by
> highmem pages during migration of CMA and fixed.
> 
> Quote from [1]'s description
> "
>     The filesystem layer expects pages in the block device's mapping to not
>     be in highmem (the mapping's gfp mask is set in bdget()), but CMA can
>     currently replace lowmem pages with highmem pages, leading to crashes in
>     filesystem code such as the one below:
> 
>       Unable to handle kernel NULL pointer dereference at virtual address 00000400
>       pgd = c0c98000
>       [00000400] *pgd=00c91831, *pte=00000000, *ppte=00000000
>       Internal error: Oops: 817 [#1] PREEMPT SMP ARM
>       CPU: 0    Not tainted  (3.5.0-rc5+ #80)
>       PC is at __memzero+0x24/0x80
>       ...
>       Process fsstress (pid: 323, stack limit = 0xc0cbc2f0)
>       Backtrace:
>       [<c010e3f0>] (ext4_getblk+0x0/0x180) from [<c010e58c>] (ext4_bread+0x1c/0x98)
>       [<c010e570>] (ext4_bread+0x0/0x98) from [<c0117944>] (ext4_mkdir+0x160/0x3bc)
>        r4:c15337f0
>       [<c01177e4>] (ext4_mkdir+0x0/0x3bc) from [<c00c29e0>] (vfs_mkdir+0x8c/0x98)
>       [<c00c2954>] (vfs_mkdir+0x0/0x98) from [<c00c2a60>] (sys_mkdirat+0x74/0xac)
>        r6:00000000 r5:c152eb40 r4:000001ff r3:c14b43f0
>       [<c00c29ec>] (sys_mkdirat+0x0/0xac) from [<c00c2ab8>] (sys_mkdir+0x20/0x24)
>        r6:beccdcf0 r5:00074000 r4:beccdbbc
>       [<c00c2a98>] (sys_mkdir+0x0/0x24) from [<c000e3c0>] (ret_fast_syscall+0x0/0x30)
> "
> 
> Memory-hotplug has same problem with CMA so [1]'s fix could be applied
> with memory-hotplug, too.
> 
> Fix it by reusing.

Do we think this issue should be fixed in 3.6?  Earlier?

> @@ -809,8 +802,12 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  			putback_lru_pages(&source);
>  			goto out;
>  		}
> -		/* this function returns # of failed pages */
> -		ret = migrate_pages(&source, hotremove_migrate_alloc, 0,
> +
> +		/*
> +		 * alloc_migrate_target should be improooooved!!

Not a helpful comment!  If you've identified some improvement then
please do provide all the details.

> +		 * migrate_pages returns # of failed pages.
> +		 */
> +		ret = migrate_pages(&source, alloc_migrate_target, 0,
>  							true, MIGRATE_SYNC);
>  		if (ret)
>  			putback_lru_pages(&source);

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