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SubjectRe: Minor kmemleak report via bdev_cache_init
On Tue, 06 Jan 2009 18:25:34 +0000
Catalin Marinas <catalin.marinas@arm.com> wrote:

> Hi Denis,
>
> With the commit c2acf7b908217 (fs/block_dev.c: __read_mostly improvement
> and sb_is_blkdev_sb utilization), the bd_mnt is only local and kmemleak
> reports the corresponding vfsmnt structure as unreferenced (together
> with the duplicated name) since it can no longer track a valid pointer
> to it:
>
> unreferenced object 0xdf813848 (size 128):
> comm "swapper", pid 0, jiffies 4294937384
> backtrace:
> [<c00872c0>] kmemleak_alloc+0x144/0x27c
> [<c00848c8>] kmem_cache_alloc+0x108/0x13c
> [<c009fc00>] alloc_vfsmnt+0x20/0x144
> [<c008bc34>] vfs_kern_mount+0x30/0xac
> [<c008bccc>] kern_mount_data+0x1c/0x20
> [<c00104a8>] bdev_cache_init+0x54/0x90
> [<c000fd7c>] vfs_caches_init+0xfc/0x128
> [<c0008984>] start_kernel+0x1f8/0x254
> unreferenced object 0xdf8033d0 (size 32):
> comm "swapper", pid 0, jiffies 4294937384
> backtrace:
> [<c00872c0>] kmemleak_alloc+0x144/0x27c
> [<c0085e10>] __kmalloc_track_caller+0x15c/0x194
> [<c0071704>] kstrdup+0x3c/0x58
> [<c009fc5c>] alloc_vfsmnt+0x7c/0x144
> [<c008bc34>] vfs_kern_mount+0x30/0xac
> [<c008bccc>] kern_mount_data+0x1c/0x20
> [<c00104a8>] bdev_cache_init+0x54/0x90
> [<c000fd7c>] vfs_caches_init+0xfc/0x128
> [<c0008984>] start_kernel+0x1f8/0x254
>
> Can this object be freed (as below) or should I just tell kmemleak to
> ignore it (or is it referenced and that's a kmemleak false positive)?
>
>
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 349a26c..78e469c 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -344,6 +344,7 @@ void __init bdev_cache_init(void)
> if (IS_ERR(bd_mnt))
> panic("Cannot create bdev pseudo-fs");
> blockdev_superblock = bd_mnt->mnt_sb; /* For writeback */
> + free_vfsmnt(bd_mnt);
> }
>

hm, yes, well, we might be able to get away with that - the kernel
holds onto bd_mnt->mnt_sb for internal use for all time, but we don't
directly use that vfsmount for anything after we've constructed
blockdev_superblock.

However there might well be things under blockdev_superblock which point
back at this vfsmount - dunno, I didn't check.



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