mount stuck in infinite loop

mount stuck in infinite loop

[Francesco Ruggeri]

I had a few cases of mount getting stuck in an infinite loop.
This happens when bdev->bd_inode->i_blkbits gets modified (for
example by bd_set_size()) while *_fill_super() is between
sb_min_blocksize() and sb_bread(), and the new value is inconsistent
with the block size used in fill_super().
When this happens one can get into an infinite loop in
__getblk_slow(), if grow_buffers() allocates/finds a page with a
given index, but __find_get_block{_slow}() looks for (and does not
find) a page with a different index.

I can reproduce it in 4.14.15 with the following script.
I can also reproduce it in 4.9 and 3.18.

FILE=/tmp/fsfile
MNT=/tmp/fsmnt
LOOP=`losetup -f`

rm -rf $FILE $MNT

mkdir $MNT
dd if=/dev/zero of=$FILE count=32 bs=1MiB
losetup $LOOP $FILE
mkfs -t ext4 $LOOP
while true; do losetup -D $LOOP; losetup $LOOP $FILE; done \
	2>/dev/null >/dev/null &
for ((i=0; i<100; i++)); do echo ================== $i; \
	mount $LOOP $MNT; umount $MNT; done

The issue is that __find_get_block_slow() and grow_buffers() compute
the page index in different ways.
I am not sure what the correct solution should be here.

Thanks,
Francesco Ruggeri

Re: mount stuck in infinite loop

[Francesco Ruggeri]

Does anybody have any opinions about this?

Thanks,
Francesco Ruggeri

On Wed, Jan 31, 2018 at 9:16 AM, Francesco Ruggeri <fruggeri@arista.com> wrote:
> I had a few cases of mount getting stuck in an infinite loop.
> This happens when bdev->bd_inode->i_blkbits gets modified (for
> example by bd_set_size()) while *_fill_super() is between
> sb_min_blocksize() and sb_bread(), and the new value is inconsistent
> with the block size used in fill_super().
> When this happens one can get into an infinite loop in
> __getblk_slow(), if grow_buffers() allocates/finds a page with a
> given index, but __find_get_block{_slow}() looks for (and does not
> find) a page with a different index.
>
> I can reproduce it in 4.14.15 with the following script.
> I can also reproduce it in 4.9 and 3.18.
>
> FILE=/tmp/fsfile
> MNT=/tmp/fsmnt
> LOOP=`losetup -f`
>
> rm -rf $FILE $MNT
>
> mkdir $MNT
> dd if=/dev/zero of=$FILE count=32 bs=1MiB
> losetup $LOOP $FILE
> mkfs -t ext4 $LOOP
> while true; do losetup -D $LOOP; losetup $LOOP $FILE; done \
>         2>/dev/null >/dev/null &
> for ((i=0; i<100; i++)); do echo ================== $i; \
>         mount $LOOP $MNT; umount $MNT; done
>
> The issue is that __find_get_block_slow() and grow_buffers() compute
> the page index in different ways.
> I am not sure what the correct solution should be here.
>
> Thanks,
> Francesco Ruggeri
>