linux-kernel - Re: [RFC PATCH v3] fuse: add new function to invalidate cache for all inodes

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Message-ID: <a5c3eb63-0fab-4751-af2f-8cb48c06b47f@ddn.com>
Date: Mon, 10 Feb 2025 20:06:55 +0000
From: Bernd Schubert <bschubert@....com>
To: Joanne Koong <joannelkoong@...il.com>, Luis Henriques <luis@...lia.com>,
	Miklos Szeredi <miklos@...redi.hu>
CC: "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Matt Harvey
	<mharvey@...ptrading.com>
Subject: Re: [RFC PATCH v3] fuse: add new function to invalidate cache for all
 inodes

On 2/10/25 20:33, Joanne Koong wrote:
> On 2/10/25 6:33 AM, 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 sb dcache.
>>
>> Signed-off-by: Luis Henriques <luis@...lia.com>
>> ---
>> * Changes since v2
>> Use the new helper from fuse_reverse_inval_inode(), as suggested by
>> Bernd.
>>
>> Also updated patch description as per checkpatch.pl suggestion.
>>
>> * Changes since v1
>> As suggested by Bernd, this patch v2 simply adds an helper function that
>> will make it easier to replace most of it's code by a call to function
>> super_iter_inodes() when Dave Chinner's patch[1] eventually gets merged.
>>
>> [1] https://lore.kernel.org/r/20241002014017.3801899-3-
>> david@...morbit.com
>>
>>   fs/fuse/inode.c           | 67 +++++++++++++++++++++++++++++++++++----
>>   include/uapi/linux/fuse.h |  3 ++
>>   2 files changed, 63 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
>> index e9db2cb8c150..45b9fbb54d42 100644
>> --- a/fs/fuse/inode.c
>> +++ b/fs/fuse/inode.c
>> @@ -547,25 +547,78 @@ struct inode *fuse_ilookup(struct fuse_conn *fc,
>> u64 nodeid,
>>       return NULL;
>>   }
>>   +static void inval_single_inode(struct inode *inode, struct
>> fuse_conn *fc)
>> +{
>> +    struct fuse_inode *fi;
>> +
>> +    fi = get_fuse_inode(inode);
>> +    spin_lock(&fi->lock);
>> +    fi->attr_version = atomic64_inc_return(&fc->attr_version);
>> +    spin_unlock(&fi->lock);
>> +    fuse_invalidate_attr(inode);
>> +    forget_all_cached_acls(inode);
>> +}
>> +
>> +static int fuse_reverse_inval_all(struct fuse_conn *fc)
>> +{
>> +    struct fuse_mount *fm;
>> +    struct super_block *sb;
>> +    struct inode *inode, *old_inode = NULL;
>> +
>> +    inode = fuse_ilookup(fc, FUSE_ROOT_ID, NULL);
>> +    if (!inode)
>> +        return -ENOENT;
>> +
>> +    fm = get_fuse_mount(inode);
> 
> I think if you pass in &fm as the 3rd arg to fuse_ilookup(), it'll pass
> back the fuse mount and we won't need get_fuse_mount().
> 
>> +    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) {
>> +        spin_lock(&inode->i_lock);
>> +        if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
>> +            !atomic_read(&inode->i_count)) {
> 
> Will inode->i_count ever be 0? AFAIU, inode->i_count tracks the inode
> refcount, so if this is 0, doesn't this mean it wouldn't be on the sb-
>>s_inodes list?
> 
>> +            spin_unlock(&inode->i_lock);
>> +            continue;
>> +        }
>> +
>> +        __iget(inode);
>> +        spin_unlock(&inode->i_lock);
>> +        spin_unlock(&sb->s_inode_list_lock);
> 
> Maybe worth adding a comment here since there can be inodes added after
> the s_inode_list_lock is dropped and before it's acquired again that
> when inodes get added to the head of sb->s_inodes, it's always for I_NEW
> inodes.
> 
>> +        iput(old_inode);
> 
> Maybe a dumb question but why is old_inode needed? Why can't iput()just
> be called right after inval_single_inode()?

I had wondered the same in v1. Issue is that there is a list iteration
that releases the locks - if the put would be done immediately it could
not continue on "old_inode" as it might not exist anymore.



> 
>> +
>> +        inval_single_inode(inode, fc);
>> +
>> +        old_inode = inode;
>> +        cond_resched();
> 
> Could you explain why a cond_resched() is needed here?

Give other threads a chance to work? The list might be huge?



Thanks,
Bernd