Re: [PATCH 1/1] ext4: fix crash on BUG_ON in ext4_alloc_group_tables
From: Baokun Li
Date: Thu Sep 26 2024 - 05:40:35 EST
On 2024/9/26 17:23, Aleksandr Mikhalitsyn wrote:
On Thu, Sep 26, 2024 at 10:50 AM Baokun Li <libaokun1@xxxxxxxxxx> wrote:
On 2024/9/26 0:17, Aleksandr Mikhalitsyn wrote:
On Wed, Sep 25, 2024 at 5:57 PM Jan Kara <jack@xxxxxxx> wrote:
On Wed 25-09-24 16:33:24, Alexander Mikhalitsyn wrote:
[ 33.882936] EXT4-fs (dm-5): mounted filesystem 8aaf41b2-6ac0-4fa8-b92b-77d10e1d16ca r/w with ordered data mode. Quota mode: none.
[ 33.888365] EXT4-fs (dm-5): resizing filesystem from 7168 to 786432 blocks
[ 33.888740] ------------[ cut here ]------------
[ 33.888742] kernel BUG at fs/ext4/resize.c:324!
Ah, I was staring at this for a while before I understood what's going on
(it would be great to explain this in the changelog BTW). As far as I
understand commit 665d3e0af4d3 ("ext4: reduce unnecessary memory allocation
in alloc_flex_gd()") can actually make flex_gd->resize_bg larger than
flexbg_size (for example when ogroup = flexbg_size, ngroup = 2*flexbg_size
- 1) which then confuses things. I think that was not really intended and
Hi Jan,
First of all, thanks for your reaction/review on this one ;-)
You are absolutely right, have just checked with our reproducer and
this modification:
diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index e04eb08b9060..530a918f0cab 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -258,6 +258,8 @@ static struct ext4_new_flex_group_data
*alloc_flex_gd(unsigned int flexbg_size,
flex_gd->resize_bg = 1 << max(fls(last_group - o_group + 1),
fls(n_group - last_group));
+ BUG_ON(flex_gd->resize_bg > flexbg_size);
+
flex_gd->groups = kmalloc_array(flex_gd->resize_bg,
sizeof(struct ext4_new_group_data),
GFP_NOFS);
and yes, it crashes on this BUG_ON. So it looks like instead of making
flex_gd->resize_bg to be smaller
than flexbg_size in most cases we can actually have an opposite effect
here. I guess we really need to fix alloc_flex_gd() too.
instead of fixing up ext4_alloc_group_tables() we should really change
the logic in alloc_flex_gd() to make sure flex_gd->resize_bg never exceeds
flexbg size. Baokun?
At the same time, if I understand the code right, as we can have
flex_gd->resize_bg != flexbg_size after
5d1935ac02ca5a ("ext4: avoid online resizing failures due to oversized
flex bg") and
665d3e0af4d3 ("ext4: reduce unnecessary memory allocation in alloc_flex_gd()")
we should always refer to flex_gd->resize_bg value which means that
ext4_alloc_group_tables() fix is needed too.
Am I correct in my understanding?
Hi Alex,
Hi Baokun,
These two are not exactly equivalent.
The flex_gd->resize_bg is only used to determine how many block groups we
allocate memory to, i.e., the maximum number of block groups per resize.
And the flexbg_size is used to make some judgement on flexible block
groups, for example, the BUG_ON triggered in the issue is to make sure
src_group and last_group must be in the same flexible block group.
Huge thanks for explaining this!
Then I guess it's better if you send a patch with your fix.
Feel free to add my Tested-by tag.
Okay, I'll send a patch later.
Question to you and Jan. Do you guys think that it makes sense to try
to create a minimal reproducer for this problem without Incus/LXD involved?
(only e2fsprogs, lvm tools, etc)
I guess this test can be put in the xfstests test suite, right?
Kind regards,
Alex
I think it makes sense, and it's good to have more use cases to look
around some corners. If you have an idea, let it go.
Regards, Baokun