Re: [PATCH v2 10/17] ovl: decode lower file handles of unlinked but open files

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On Tue, Jan 16, 2018 at 11:16 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Thu, Jan 4, 2018 at 6:20 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>> Lookup overlay inode in cache by origin inode, so we can decode a file
>> handle of an open file even if the index has a whiteout index entry to
>> mark this overlay inode was unlinked.
>>
>> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
>> ---
>>  fs/overlayfs/export.c    | 22 ++++++++++++++++++++--
>>  fs/overlayfs/inode.c     | 16 ++++++++++++++++
>>  fs/overlayfs/overlayfs.h |  1 +
>>  3 files changed, 37 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
>> index 602bada474ba..6ecb54d4b52c 100644
>> --- a/fs/overlayfs/export.c
>> +++ b/fs/overlayfs/export.c
>> @@ -385,13 +385,21 @@ static struct dentry *ovl_lower_fh_to_d(struct super_block *sb,
>>         struct ovl_path *stack = &origin;
>>         struct dentry *dentry = NULL;
>>         struct dentry *index = NULL;
>> +       struct inode *inode = NULL;
>> +       bool is_deleted = false;
>>         int err;
>>
>>         /* First lookup indexed upper by fh */
>
> Why not first look up origin, then look up ovl inode by origin?  It
> seems a faster path than going through the index first.  Obviously if
> icache lookup fails then we need to look up index, but the common case
> will the cached one, so that should be the fast one, no?
>

Not really, because we do not know if the file handle is dir or non-dir.
If file handle is dir than decode of file handle is expensive and can
reduce worst case from two file handle decodes to just one:

For lower dir:
- one index lookup fails
- one lower dir decode
- one icache lookup
- maybe one ovl_lookup_real(is_upper=false)

For copied up indexed dir:
- one index lookup success
- one upper dir decode
- one ovl_lookup_real(is_upper=true)

That method avoids the origin dir decode for upper indexed
dir at the cost of not looking for the decoded dir in icache.

How about this as in idea: hash overlay inodes for NFS export
by origin fh instead of by origin inode pointer.

We can also avoid the "lookup index first" for non-dir
if we set a flag OVL_FH_FLAG_CONNECTABLE on exported
dir file handle, but my thinking was trying to keep the first version
simple with as fewer special cases as possible.

Thanks,
Amir.
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