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Date:   Tue, 06 Feb 2018 00:24:21 -0200
From:   Gabriel Krisman Bertazi <krisman@...labora.co.uk>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     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
Subject: Re: [PATCH RFC v2 00/13] NLS/UTF-8 Case-Insensitive lookups for ext4 and VFS proposal

Al Viro <viro@...IV.linux.org.uk> writes:

> On Thu, Jan 25, 2018 at 12:53:36AM -0200, Gabriel Krisman Bertazi wrote:
>
>> The second proposal is related to the VFS layer:
>> 
>> (2) Enable Insensitive lookup support on a per-mountpoint basis,
>> via a MS_CASEFOLD flag, with the ultimate goal of supporting a
>> case-insensitive bind mount of a subtree, side-by-side with a
>> sensitive version of the filesystem.
>
> First reaction: No.  With the side of HELL NO.
>
> Your ultimate goal sounds utterly insane - dcache tree must be shared
> for all mounts.  Moreover, "would these two names refer to the same
> object" can not be mount-dependent.  Not going to happen.
>
> Please, post the description of what you are planning to do.
> Detailed.  I'm not saying that it's 100% impossible to do correctly,
> but I'm _very_ sceptical about the feasibility.
>
> I'm certainly not going to ACK any VFS changes until you convince
> me that this thing can be done with sane semantics.

Hi Viro,

Sorry for the delay.  I wanted to get a better understanding of the
code before getting back to you.

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.

After learning, last year, about the similar Android issue from Ted at a
conference, I started to believe a bind mount is the appropriate
solution for both our use cases.

Like Ted mentioned, we let the user shot herself in the foot by having
two files with the exact CI name, and which one she gets will be
undefined.  My current solution gets the exact match if it is present,
and my second approach could be adapted to do that with a performance
cost only if both files are not cached.  That seem to be acceptable by
Ted and shouldn't be a problem for me too.

Let me start by discussing what I have now, which is a functional
solution, to the best of my knowledge, but which still requires rework
to better benefit from the dentry cache.  Then I will discuss the
solutions I am working on.

(1) What I have now:

As I mentioned in the first email, MS_CASEFOLD triggers a MNT_CASEFOLD
flag in the vfsmount structure.  This flag, which gets recalculated
every time a mountpoint is traversed, is used to decide if we do a
casefolded ->lookup() for each component of the path.  This obviously
allows paths with multiple nested mountpoints to work.

The ->lookup() function calls d_add_ci(), making sure that only the
exact-match dentry is inserted in the cache.  This means that we don't
have multiple dentries, differing only by case, associated to the same
file.  This is an important semantic, which I am not modifying.  This
also means that a following CI lookup with an inexact-case match will
trigger another ->lookup(), which is bad, but is manageable (for now).
I address this problem later in this email.

Regarding negative dentries.  If we have a negative dentry in the cache,
the case-sensitive algorithm will abort the lookup and return the
dentry, nothing new here.  If, on the other hand, That component lookup
is under a MNT_CASEFOLD search, the code does a bigger effort, ignores
the negative dentry and still do ->lookup().

This means that:

 - Case-sensitive (CS) search performance is not affected, except for
   the following sequence: Do a CS(foo) which returns a negative_dentry,
   then CI(FOO), and finally CS(foo).  This will require the second
   CS(foo) to re-do the search and recreate the negative dentry.  If
   only CS searches are done, the performance should be unaffected.

 - Case-insensitive (CI) benefits from the dentry cache as long as it is
 doing an exact-case name match.  Otherwise, it always falls to
 ->lookup().  The exact-name dentry, if already in the cache, will be
 returned in both cases.

 - Negative dentries are ignored if doing CI and it always re-do the
   search.

The prototype code is in the following branch.  I can send
patches if you want.

 https://gitlab.collabora.com/krisman/linux.git -b vfs-ms_casefold

(2) What I want to do:

I want to allow lookups of the inexact name to find the cached dentry,
preventing it from going to the disk only to find out later it already
has that dentry cached in some other cache bucket.  This means that,
whenever there is a MNT_CASEFOLD enabled, every mountpoint of that
superblock needs to use d_hash(casefold(name)), such that every file is
in a known bucket.  Obviously this is a worse hash function, but the
cost is only payed by filesystems using the feature.  If a bind mount is
done, we must rehash all existing entries of that filesystem at
"bind,remount,ignorecase" time.

Like the previous version, we only store exact name dentries, even
though the bucket is different now.  When doing a LOOKUP_CASEFOLD, we
use a different ->d_compare_CI() to do the matching, such that we can
find the right dentry or not, in both scenarios (CI and CS).

We still can't trust negative dentries when doing a CI lookup. Consider
the case where we do CS(FOO) and foo doesn't exist. Now we have a
negative_dentry(FOO), but CI(FOO) would still need to find an existing
'foo' file.  The first solution is to make fast_CI_lookup() walk the
entire bucket and ignore previous matching negative dentries if a
positive one is found.  If no positive dentry is found, then fallback
and do ->lookup().  A superior solution is to mark negative dentries as
soft negatives and hard negatives, depending on what kind of lookup
created the dentry, whether CS or CI, respectively.  Then, we can allow
a CI lookup to return the negative dentry only if it is a HARD negative,
and try the ->lookup() otherwise.  For this to work, any dentry creation
on that parent requires that we invalidate hard negative children
dentries, and make them soft, at insertion/rename time.

With these changes I believe the dentry cache could be shared by code
doing CI and CS lookups of the same dentry.  My main concern is whether
we could invalidate a negative dentry without harming parallel lookups
using that dentry, but I believe I can work around that.

I believe this covers everything I could identify, but please let me
know if I missed something, obvious or not.

-- 
Gabriel Krisman Bertazi

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