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Message-ID: <dcda93dd-f2ef-4419-ae73-7d3c55b5df8f@huawei.com> Date: Thu, 26 Sep 2024 16:50:41 +0800 From: Baokun Li <libaokun1@...wei.com> To: Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>, Jan Kara <jack@...e.cz> CC: <tytso@....edu>, <stable@...r.kernel.org>, Andreas Dilger <adilger.kernel@...ger.ca>, Stéphane Graber <stgraber@...raber.org>, Christian Brauner <brauner@...nel.org>, <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>, <linux-ext4@...r.kernel.org>, Wesley Hershberger <wesley.hershberger@...onical.com>, Yang Erkun <yangerkun@...wei.com> Subject: Re: [PATCH 1/1] ext4: fix crash on BUG_ON in ext4_alloc_group_tables On 2024/9/26 0:17, Aleksandr Mikhalitsyn wrote: > On Wed, Sep 25, 2024 at 5:57 PM Jan Kara <jack@...e.cz> 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, 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. Regards, Baokun
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