Re: [PATCH] cramfs corruption after BLKFLSBUF on loop device

[Andrew Morton]

On Wed, 20 Sep 2006 15:20:11 +0200
Olaf Hering <olh@xxxxxxx> wrote:

> On Thu, Jun 01, Olaf Hering wrote:
> 
> > ...
> > Error -3 while decompressing!
> > c0000000009592a2(2649)->c0000000edf87000(4096)
> > Error -3 while decompressing!
> > c000000000959298(2520)->c0000000edbc7000(4096)
> > Error -3 while decompressing!
> > c000000000959c70(2489)->c0000000f1482000(4096) 
> > Error -3 while decompressing!
> > c00000000095a629(2355)->c0000000edaff000(4096)
> > Error -3 while decompressing!
> > ...
> 
> Today I looked at this bug again and found that 2.6.18-rc6-git2 has
> fix for this. Is the patch below supposed to fix the cramfs corruption
> or does it just paper over the bug?
> 
> ...
> cramfs_read() clears parts of the src buffer because the page is not
> uptodate. invalidate_bdev() called from block_ioctl(BLKFLSBUF) will set
> ClearPageUptodate() after cramfs_read() got the page from read_cache_page()
> ...
> 
> /root/cramfscrash.sh
> #!/bin/bash
> # cd /dev/shm/
> # tar xfz /mounts/mirror/kernel/v2.6/linux-2.6.18.tar.gz
> # cd linux-2.6.18/
> # mkfs.cramfs drivers /tmp/cramfs.image
> mount -vnt proc proc /proc
> echo 1 > /proc/sys/kernel/panic
> echo 9 > /proc/sysrq-trigger

That's a novel way of setting the log level.

> mount -vnt sysfs sysfs /sys
> modprobe -v loop
> mount -vnt cramfs -o loop /tmp/cramfs.image /mnt
> while :;do /sbin/blockdev --flushbufs /dev/loop0;done </dev/null &>/dev/null&
> while :;do /usr/bin/find /mnt -type f -print0|xargs -0 cat &>/dev/null;done
> 
> 
> kernel cmdline
> xmon=off panic=1 sysrq=1 quiet root=/dev/disk/by-uuid/d50e4029-2e91-4332-bb16-24f946a74d3f ro init=/root/cramfscrash.sh
> 
> 
> 
>  016eb4a0ed06a3677d67a584da901f0e9a63c666.patch
> From: Andrew Morton <akpm@xxxxxxxx>
> 
> If a CPU faults this page into pagetables after invalidate_mapping_pages()
> checked page_mapped(), invalidate_complete_page() will still proceed to remove
> the page from pagecache.  This leaves the page-faulting process with a
> detached page.  If it was MAP_SHARED then file data loss will ensue.
> 
> Fix that up by checking the page's refcount after taking tree_lock.

Yes, I think this is a reasonable solution to the cramfs race.  Previously,
invalidate_inode_pages() was removing pages from pagecache even if some
other thread had a ref against them, which is pretty bad behaviour.
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