Re: [RFC PATCH] fuse: add new function to invalidate cache for all inodes

[Luis Henriques]

Hi Bernd,

On Wed, Jan 15 2025, Bernd Schubert wrote:

> On 1/15/25 17:32, Luis Henriques wrote:
>> Currently userspace is able to notify the kernel to invalidate the cache
>> for an inode.  This means that, if all the inodes in a filesystem need to
>> be invalidated, then userspace needs to iterate through all of them and do
>> this kernel notification separately.
>> 
>> This patch adds a new option that allows userspace to invalidate all the
>> inodes with a single notification operation.  In addition to invalidate all
>> the inodes, it also shrinks the superblock dcache.
>
> Out of interest, what is the use case?

This is for a read-only filesystem.  However, the filesystem objects
(files, directories, ...) may change dramatically in an atomic way, so
that a totally different set of objects replaces the old one.

Obviously, this patch would help with the process of getting rid of the
old generation of the filesystem.

>> 
>> Signed-off-by: Luis Henriques <luis@xxxxxxxxxx>
>> ---
>> Just an additional note that this patch could eventually be simplified if
>> Dave Chinner patch to iterate through the superblock inodes[1] is merged.
>> 
>> [1] https://lore.kernel.org/r/20241002014017.3801899-3-david@xxxxxxxxxxxxx
>> 
>>  fs/fuse/inode.c           | 53 +++++++++++++++++++++++++++++++++++++++
>>  include/uapi/linux/fuse.h |  3 +++
>>  2 files changed, 56 insertions(+)
>> 
>> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
>> index 3ce4f4e81d09..1fd9a5f303da 100644
>> --- a/fs/fuse/inode.c
>> +++ b/fs/fuse/inode.c
>> @@ -546,6 +546,56 @@ struct inode *fuse_ilookup(struct fuse_conn *fc, u64 nodeid,
>>  	return NULL;
>>  }
>>  
>> +static int fuse_reverse_inval_all(struct fuse_conn *fc)
>> +{
>> +	struct fuse_mount *fm;
>> +	struct super_block *sb;
>> +	struct inode *inode, *old_inode = NULL;
>> +	struct fuse_inode *fi;
>> +
>> +	inode = fuse_ilookup(fc, FUSE_ROOT_ID, NULL);
>> +	if (!inode)
>> +		return -ENOENT;
>> +
>> +	fm = get_fuse_mount(inode);
>> +	iput(inode);
>> +	if (!fm)
>> +		return -ENOENT;
>> +	sb = fm->sb;
>> +
>> +	spin_lock(&sb->s_inode_list_lock);
>> +	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
>
> Maybe list_for_each_entry_safe() and then you can iput(inode) before the
> next iteration?

I can rework this loop, but are you sure it's safe to use that?  (Genuine
question!)

I could only find two places where list_for_each_entry_safe() is being
used to walk through the sb inodes.  And they both use an auxiliary list
that holds the inodes to be processed later.  All other places use the
pattern I'm following here.

Or did I misunderstood your suggestion?

Cheers,
-- 
Luís


>> +		spin_lock(&inode->i_lock);
>> +		if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
>> +		    !atomic_read(&inode->i_count)) {
>> +			spin_unlock(&inode->i_lock);
>> +			continue;
>> +		}
>> +
>> +		__iget(inode);
>> +		spin_unlock(&inode->i_lock);
>> +		spin_unlock(&sb->s_inode_list_lock);
>> +		iput(old_inode);
>> +
>> +		fi = get_fuse_inode(inode);
>> +		spin_lock(&fi->lock);
>> +		fi->attr_version = atomic64_inc_return(&fm->fc->attr_version);
>> +		spin_unlock(&fi->lock);
>> +		fuse_invalidate_attr(inode);
>> +		forget_all_cached_acls(inode);
>> +
>> +		old_inode = inode;
>> +		cond_resched();
>> +		spin_lock(&sb->s_inode_list_lock);
>> +	}
>> +	spin_unlock(&sb->s_inode_list_lock);
>> +	iput(old_inode);
>
>
> Thanks,
> Bernd