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Date:   Tue, 13 Feb 2018 06:43:10 +0800
From:   Gao Xiang <hsiangkao@....com>
To:     Gabriel Krisman Bertazi <krisman@...labora.co.uk>,
        Gao Xiang <gaoxiang25@...wei.com>
Cc:     Al Viro <viro@...IV.linux.org.uk>, tytso@....edu,
        david@...morbit.com, olaf@....com, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, alvaro.soliverez@...labora.co.uk,
        kernel@...ts.collabora.co.uk, hutj <hutj@...wei.com>
Subject: Re: [PATCH RFC v2 00/13] NLS/UTF-8 Case-Insensitive lookups for ext4
 and VFS proposal

Hi Gabriel,

Thanks for your reply.

On 2018/2/13 3:56, Gabriel Krisman Bertazi wrote:
> Gao Xiang <gaoxiang25@...wei.com> writes:
>
>> Could I express my opinion? I have working on case-insensitive sdcardfs
>> for months.
> Hi Gao,
>
> Thanks for helping out with this topic.
>
>> I think your problem is how we optimise a case-insensitive lookup on the
>> file system with a case-sensitive dcache (I mean d_add and no d_compare
>> and d_hash).
> Are d_compare and d_hash to be considered really disruptive
> performance-wise?  Even if they are only used when casefold/encoding
> support is enabled?  I don't see how we could better use the dcache
> without at least requiring these functions to handle CI cases.
I mean if both "Android" and "anDroid" exist in the same directory of 
on-disk ext4,
I could tend to make both "Android" and "anDroid" accessed by
the case-exact name lookup, otherwise do a case-insensitive lookup. eg. 
(my current implementation),
1. open("/mnt/anDroid", O_EXCL|O_CREAT)
2. open("/mnt/Android", O_EXCL|O_CREAT)
3. open("/mnt-ci/Android") --- exactly Android
4. open("/mnt-ci/anDroid") --- exactly anDroid
5. open("/mnt-ci/ANDROID") --- Android or anDroid, do a case-insensitive 
lookup.
6. unlink("/mnt-ci/Android") --- exactly Android
7. unlink("/mnt-ci/ANDROID") --- anDroid, do a case-insensitive lookup.

>> In that case, we could not trust the negative dentry when _creating_ a
>> case-insensitive file, for example:
>>     there exists "anDroid" on-disk, but ext4's in-memory dcache only has
>> the negative "Android", if we lookup "Android" we will get the
>> _negative_ dentry, but we _cannot_ create it since "anDroid" exists on
>> disk. In the create case, an on-disk _iterate_ (or readdir) is
>> necessary.
> In my previous email, I mentioned my current implementation ignores
> negative dentries and forces a ->lookup(), which walks over the disk
> entries.  (I had to add a fix to the creation path in the vfs-ms_casefold
> branch to exactly match that description, so you might have missed the
> updated version in that branch).
I am sorry, I haven't looked into your CI patch yet.

I decided to join in this topic because in the past months, I did some work
on the case-insensitive sdcardfs, and I just want to express my thoughts 
for this topic.

I don't know which level negative dentries you ignore, your 
case-insensitive negative dentries
or the original underlayfs(eg. ext4) case-sensitive negative dentries?

Actually I observed some false "negative dentries" race or deadlock
if multiple case-insensitive mounts exist and do fs-ops on these mounts 
at the same time,
but I am not sure whether your implementation has these potential issues 
in principle or not.

In addition, quote `
Our customer is interested in exposing a subtree of an existing
filesystem (native Linux filesystems, xfs, ext4 and others) in an
case-insensitive lookup manner, without paying the cost of a userspace
getdents implementation, and, preferably, without requiring the user to
modify data on the disk.`

I think the *most expensive* operation for case-insensitive lookup is to
create a not-exist file rather than do a existed-file lookup.
It takes much overhead and I think no straightforward way to directly 
reduce the cost
and improve its performance.

>
> Either way, this case is supported like this:
>
> If we have two bind-mounts of the same directory, /mnt and /mnt-ci,
> case-sensitive and case-insensitive, respectively,  We can do:
>
> open("/mnt/anDroid", O_EXCL|O_CREAT) = 3
> open("/mnt/Android", 0) = -2 No such file or directory
> open("/mnt-ci/Android", 0) = 4
> open("/mnt-ci/Android", O_EXCL|O_CREAT) = -17 File exists
> open("/mnt-ci/AndROID", O_EXCL|O_CREAT) = -17 File exists
>
> The second open() is expected to create an negative_dentry of "Android",
> which, if it wasn't ignored by the 3th open(), the CI operation would
> have failed.  Notice that the 3th open() operation actually opens the
Yes, the 3th open() should invalidate the 2nd negative dentry.
However, I don't know your detailed implementation behavior, for example 
as follows:

1. open("/mnt/anDroid", O_EXCL|O_CREAT)
2. open("/mnt/Android", 0)                 ---- dentry A
3. open("/mnt-ci/Android", 0)             ---- CI-dentry B or the same 
dentry A?

if 2 and 3 are the different denties but the same inode, furthermore,
if the inode is a directory inode, it seems like a hard link directory,
any *potential deadlock* with that?

and how about?
1. open("/mnt/anDroid", O_EXCL|O_CREAT)             --- inode A
2. close("/mnt/anDroid") --- still positive, referenced
3. open("/mnt/Android", 0) --- No such file or directory
4. open("/mnt-ci/Android", 0) --- positive, inode A
5. close("/mnt-ci/Android") --- still positive, referenced
6. open("/mnt/android", O_EXCL|O_CREAT)             --- inode B
7. unlink("/mnt/anDroid")   --- expected behavior? since we have 2) and 
5) referenced, can the inode finally be evicted? ---
8. open("/mnt-ci/Android", 0) --- ??? positive and inode B? ---

> file that was created by the first open().  It doesn't create a new
> file.
>
> Following on, the 4th operation (file creation) *must fail* because
> there is a CI name collision with /mnt-ci/anDroid.  The same is true for
> the final case.
>
>> I could give another example, if we uses case-insentive ext4 and create
>> "Android" and "anDroid", how to deal with the case in the
>> case-insensitive way?
>>     I mean in that case we should make both "Android" and "anDroid" can
>> access, right?
> Not sure if I follow you here, but I'm assuming we create Android and
> anDroid in the sensitive mountpoint, because, otherwise the
> second file creation in the insensitive mountpoint would fail.
>
> This is the case where I'm hiding one of the previously (CS) created
> files, when in the insensitive mountpoint, and the user is shooting
> himself.  For the sensitive case, Both stays visible to the user.
As I mentioned above....
1. open("/mnt/anDroid", O_EXCL|O_CREAT)
2. open("/mnt/Android", O_EXCL|O_CREAT)
3. open("/mnt-ci/Android") --- exactly "Android"
4. open("/mnt-ci/anDroid") --- exactly "anDroid"
5. open("/mnt-ci/ANDROID") --- "Android" or "anDroid", do a 
case-insensitive lookup.
6. unlink("/mnt-ci/Android") --- exactly "Android"
7. unlink("/mnt-ci/ANDROID") --- "anDroid", do a case-insensitive lookup.

>>     I think we need to build a special case-sensitive dcache rather than
>> a case-insensitive dcache following the native case-insentive fs(use
>> d_add_ci, d_compare and d_hash, eg. fat, ntfs...)
> What do you think about the second part of my proposal, where I mention
> dealing differently with negative dentries created by a CI lookup?
> We don't need to ignore them if we can invalidate them after a creation
> in the directory.
I feel some complex of your second part... I still need some time to 
look into that...
and I think it is useless of negative dentries for that case 
intuitively.....
>
>> Finally, I agree "let the user shot herself in the foot by having two
>> files with the exact CI name", but I think it could not the VFS
>> _busniess_ itself since each customer solution "case-sensitive ext4 ->
>> case-insensitive lookup" has their _perfered_ way (for example,
>> "android" and "Android" exist, A perfers android and B perfers Android.
> I don't see how we could defer the decision to the filesystem, that's a
> pretty good problem, which I don't have a solution right now.
>
>> Finally, I think for optmization, ext4 or other fs could add some dir
>> inode _tag_ and supports native case-insensitive for these dirs could be
>> better....
> Agreed. But I'm seeing this as outside the scope of my proposal, since it
> is specific to each filesystem.  My ext4 adaptation, for instance, falls
> back to linear search when it can't find the exact match.
>
> Thanks,
>
I could miss something important, if I recall, I will reply in the 
future....

Thanks,

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