Re: [PATCH v2] erofs: reliably distinguish block based and fscache mode

[Jingbo Xu]

On 4/18/24 11:36 AM, Baokun Li wrote:
> On 2024/4/18 10:16, Jingbo Xu wrote:
>> Hi Baokun,
>>
>> Thanks for catching this and move forward fixing this!
> 
> Hi Jingbo，
> 
> Thanks for your review！
> 
>>
>> On 4/17/24 2:55 PM, Baokun Li wrote:
>>> When erofs_kill_sb() is called in block dev based mode, s_bdev may
>>> not have
>>> been initialised yet, and if CONFIG_EROFS_FS_ONDEMAND is enabled, it
>>> will
>>> be mistaken for fscache mode, and then attempt to free an anon_dev
>>> that has
>>> never been allocated, triggering the following warning:
>>>
>>> ============================================
>>> ida_free called for id=0 which is not allocated.
>>> WARNING: CPU: 14 PID: 926 at lib/idr.c:525 ida_free+0x134/0x140
>>> Modules linked in:
>>> CPU: 14 PID: 926 Comm: mount Not tainted 6.9.0-rc3-dirty #630
>>> RIP: 0010:ida_free+0x134/0x140
>>> Call Trace:
>>>   <TASK>
>>>   erofs_kill_sb+0x81/0x90
>>>   deactivate_locked_super+0x35/0x80
>>>   get_tree_bdev+0x136/0x1e0
>>>   vfs_get_tree+0x2c/0xf0
>>>   do_new_mount+0x190/0x2f0
>>>   [...]
>>> ============================================
>>>
>>> Instead of allocating the erofs_sb_info in fill_super() allocate it
>>> during erofs_get_tree() and ensure that erofs can always have the info
>>> available during erofs_kill_sb().
>>
>> I'm not sure if allocating erofs_sb_info in erofs_init_fs_context() will
>> be better, as I see some filesystems (e.g. autofs) do this way.  Maybe
>> another potential advantage of doing this way is that erofs_fs_context
>> is not needed anymore and we can use sbi directly.
> Yes, except for some extra memory usage when remounting,
> this idea sounds great. Let me send a version of v3 to get rid
> of erofs_fs_context.

I'm not sure if Gao Xaing also prefers this.  I think it would be better
to wait and listen for his thoughts before we sending v3.

>>
>>> Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
>>> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
>>> ---
>>> Changes since v1:
>>>    Allocate and initialise fc->s_fs_info in erofs_fc_get_tree()
>>> instead of
>>>    modifying fc->sb_flags.
>>>
>>> V1:
>>> https://lore.kernel.org/r/20240415121746.1207242-1-libaokun1@xxxxxxxxxx/
>>>
>>>   fs/erofs/super.c | 51 ++++++++++++++++++++++++++----------------------
>>>   1 file changed, 28 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
>>> index b21bd8f78dc1..4104280be2ea 100644
>>> --- a/fs/erofs/super.c
>>> +++ b/fs/erofs/super.c
>>> @@ -581,8 +581,7 @@ static const struct export_operations
>>> erofs_export_ops = {
>>>   static int erofs_fc_fill_super(struct super_block *sb, struct
>>> fs_context *fc)
>>>   {
>>>       struct inode *inode;
>>> -    struct erofs_sb_info *sbi;
>>> -    struct erofs_fs_context *ctx = fc->fs_private;
>>> +    struct erofs_sb_info *sbi = EROFS_SB(sb);
>>>       int err;
>>>         sb->s_magic = EROFS_SUPER_MAGIC;
>>> @@ -590,19 +589,6 @@ static int erofs_fc_fill_super(struct
>>> super_block *sb, struct fs_context *fc)
>>>       sb->s_maxbytes = MAX_LFS_FILESIZE;
>>>       sb->s_op = &erofs_sops;
>>>   -    sbi = kzalloc(sizeof(*sbi), GFP_KERNEL);
>>> -    if (!sbi)
>>> -        return -ENOMEM;
>>> -
>>> -    sb->s_fs_info = sbi;
>>> -    sbi->opt = ctx->opt;
>>> -    sbi->devs = ctx->devs;
>>> -    ctx->devs = NULL;
>>> -    sbi->fsid = ctx->fsid;
>>> -    ctx->fsid = NULL;
>>> -    sbi->domain_id = ctx->domain_id;
>>> -    ctx->domain_id = NULL;
>>> -
>>>       sbi->blkszbits = PAGE_SHIFT;
>>>       if (erofs_is_fscache_mode(sb)) {
>>>           sb->s_blocksize = PAGE_SIZE;
>>> @@ -704,11 +690,32 @@ static int erofs_fc_fill_super(struct
>>> super_block *sb, struct fs_context *fc)
>>>       return 0;
>>>   }
>>>   -static int erofs_fc_get_tree(struct fs_context *fc)
>>> +static void erofs_ctx_to_info(struct fs_context *fc)
>>>   {
>>>       struct erofs_fs_context *ctx = fc->fs_private;
>>> +    struct erofs_sb_info *sbi = fc->s_fs_info;
>>> +
>>> +    sbi->opt = ctx->opt;
>>> +    sbi->devs = ctx->devs;
>>> +    ctx->devs = NULL;
>>> +    sbi->fsid = ctx->fsid;
>>> +    ctx->fsid = NULL;
>>> +    sbi->domain_id = ctx->domain_id;
>>> +    ctx->domain_id = NULL;
>>> +}
>> I'm not sure if abstracting this logic into a seperate helper really
>> helps understanding the code as the logic itself is quite simple and
>> easy to be understood. Usually it's a hint of over-abstraction when a
>> simple helper has only one caller.
>>
> Static functions that have only one caller are compiled inline, so we
> don't have to worry about how that affects the code.
> 
> The reason these codes are encapsulated in a separate function is so
> that the code reader understands that these codes are integrated
> as a whole, and that we shouldn't have to move one or two of these
> lines individually.
> 
> But after we get rid of erofs_fs_context, those won't be needed
> anymore.

Yeah, I understand. It's only coding style concerns.



-- 
Thanks,
Jingbo