Re: [PATCH 2/3] sysctl: add support for drop_caches for individual filesystem

[Benjamin Coddington]

On 10 Oct 2024, at 8:16, Jan Kara wrote:

> On Thu 10-10-24 19:25:42, Ye Bin wrote:
>> From: Ye Bin <yebin10@xxxxxxxxxx>
>>
>> In order to better analyze the issue of file system uninstallation caused
>> by kernel module opening files, it is necessary to perform dentry recycling
>
> I don't quite understand the use case you mention here. Can you explain it
> a bit more (that being said I've needed dropping caches for a particular sb
> myself a few times for debugging purposes so I generally agree it is a
> useful feature).
>
>> on a single file system. But now, apart from global dentry recycling, it is
>> not supported to do dentry recycling on a single file system separately.
>> This feature has usage scenarios in problem localization scenarios.At the
>> same time, it also provides users with a slightly fine-grained
>> pagecache/entry recycling mechanism.
>> This patch supports the recycling of pagecache/entry for individual file
>> systems.
>>
>> Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
>> ---
>>  fs/drop_caches.c   | 43 +++++++++++++++++++++++++++++++++++++++++++
>>  include/linux/mm.h |  2 ++
>>  kernel/sysctl.c    |  9 +++++++++
>>  3 files changed, 54 insertions(+)
>>
>> diff --git a/fs/drop_caches.c b/fs/drop_caches.c
>> index d45ef541d848..99d412cf3e52 100644
>> --- a/fs/drop_caches.c
>> +++ b/fs/drop_caches.c
>> @@ -77,3 +77,46 @@ int drop_caches_sysctl_handler(const struct ctl_table *table, int write,
>>  	}
>>  	return 0;
>>  }
>> +
>> +int drop_fs_caches_sysctl_handler(const struct ctl_table *table, int write,
>> +				  void *buffer, size_t *length, loff_t *ppos)
>> +{
>> +	unsigned int major, minor;
>> +	unsigned int ctl;
>> +	struct super_block *sb;
>> +	static int stfu;
>> +
>> +	if (!write)
>> +		return 0;
>> +
>> +	if (sscanf(buffer, "%u:%u:%u", &major, &minor, &ctl) != 3)
>> +		return -EINVAL;
>
> I think specifying bdev major & minor number is not a great interface these
> days. In particular for filesystems which are not bdev based such as NFS. I
> think specifying path to some file/dir in the filesystem is nicer and you
> can easily resolve that to sb here as well.

Slight disagreement here since NFS uses set_anon_super() and major:minor
will work fine with it.  I'd prefer it actually since it avoids this
interface having to do a pathwalk and make decisions about what's mounted
where and in what namespace.

Ben