Date:   Wed, 07 Jun 2023 14:35:26 -0400
From:   Gabriel Krisman Bertazi <krisman@...e.de>
To:     viro@...iv.linux.org.uk
Cc:     brauner@...nel.org, tytso@....edu,
        linux-f2fs-devel@...ts.sourceforge.net, ebiggers@...nel.org,
        linux-fsdevel@...r.kernel.org, jaegeuk@...nel.org,
        linux-ext4@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH v2 0/7] Support negative dentries on
 case-insensitive ext4 and f2fs

Gabriel Krisman Bertazi <krisman@...e.de> writes:

> Hi,
>
> This is the v2 of the negative dentry support on case-insensitive directories.
> It doesn't have any functional changes from v1, but it adds more context and a
> comment to the dentry->d_name access I'm doing in d_revalidate, documenting
> why (i understand) it is safe to do it without protecting from the parallell
> directory changes.
>
> Please, let me know if the documentation is sufficient or if I'm missing some
> case.

Hi Al, Christian,

Wanted to ping about this and see if we can get a review from the vfs
side to get it merged.

Thank you!

>
> Retested with xfstests for ext4 and f2fs.
>
> --
> cover letter from v1.
>
> This patchset enables negative dentries for case-insensitive directories
> in ext4/f2fs.  It solves the corner cases for this feature, including
> those already tested by fstests (generic/556).  It also solves an
> existing bug with the existing implementation where old negative
> dentries are left behind after a directory conversion to
> case-insensitive.
>
> Testing-wise, I ran sanity checks to show it properly uses the created
> negative dentries, observed the expected performance increase of the
> dentry cache hit, and showed it survives the quick group in fstests on
> both f2fs and ext4 without regressions.
>
> * Background
>
> Negative dentries have always been disabled in case-insensitive
> directories because, in their current form, they can't provide enough
> assurances that all the case variations of a filename won't exist in a
> directory, and the name-preserving case-insenstive semantics
> during file creation prevents some negative dentries from being
> instantiated unmodified.
>
> Nevertheless, for the general case, the existing implementation would
> already work with negative dentries, even though they are fully
> disabled. That is: if the original lookup that created the dentry was
> done in a case-insensitive way, the negative dentry can usually be
> validated, since it assures that no other dcache entry exists, *and*
> that no variation of the file exists on disk (since the lookup
> failed). A following lookup would then be executed with the
> case-insensitive-aware d_hash and d_lookup, which would find the right
> negative dentry and use it.
>
> The first corner case arises when a case-insensitive directory has
> negative dentries that were created before the directory was flipped to
> case-insensitive.  A directory must be empty to be converted, but it
> doesn't mean the directory doesn't have negative dentry children.  If
> that happens, the dangling dentries left behind can't assure that no
> case-variation of the name exists. They only mean the exact name
> doesn't exist.  A further lookup would incorrectly validate them.
>
> The code below demonstrates the problem.  In this example $1 and $2 are
> two strings, where:
>
>       (i) $1 != $2
>      (ii) casefold($1) == casefold($2)
>     (iii) hash($1) == hash($2) == hash(casefold($1))
>
> Then, the following sequence could potentially return a ENOENT, even
> though the case-insensitive lookup should exist:
>
>   mkdir  d      <- Case-sensitive directory
>   touch  d/$1
>   touch  d/$2
>   unlink d/$1   <- leaves negative dentry  behind.
>   unlink d/$2   <- leaves *another* negative dentry behind.
>   chattr +F d   <- make 'd' case-insensitive.
>   touch  d/$1   <- Both negative dentries could match. finds one of them,
> 		   and instantiate
>   access d/$1   <- Find the other negative dentry, get -ENOENT.
>
> In fact, this is a problem even on the current implementation, where
> negative dentries for CI are disabled.  There was a bug reported by Al
> Viro in 2020, where a directory might end up with dangling negative
> dentries created during a case-sensitive lookup, because they existed
> before the +F attribute was set.
>
> It is hard to trigger the issue, because condition (iii) is hard to test
> on an unmodified kernel.  By hacking the kernel to force the hash
> collision, there are a few ways we can trigger this bizarre behavior in
> case-insensitive directories through the insertion of negative dentries.
>
> Another problem exists when turning a negative dentry to positive.  If
> the negative dentry has a different case than what is currently being
> used for lookup, the dentry cannot be reused without changing its name,
> in order to guarantee filename-preserving semantics to userspace.  We
> need to either change the name or invalidate the dentry. This issue is
> currently avoided in mainline, since the negative dentry mechanism is
> disabled.
>
> * Proposal
>
> The main idea is to differentiate negative dentries created in a
> case-insensitive context from those created during a case-sensitive
> lookup via a new dentry flag, D_CASEFOLD_LOOKUP, set by the filesystem
> the d_lookup hook.  Since the former can be used (except for the
> name-preserving issue), d_revalidate will just check the flag to
> quickly accept or reject the dentry.
>
> A different solution would be to guarantee no negative dentry exists
> during the case-sensitive to case-insensitive directory conversion (the
> other direction is safe).  It has the following problems:
>
>   1) It is not trivial to implement a race-free mechanism to ensure
>   negative dentries won't be recreated immediately after invalidation
>   while converting the directory.
>
>   2) The knowledge whether the negative dentry is valid (i.e. comes from
>   a case-insensitive lookup) is implicit on the fact that we are
>   correctly invalidating dentries when converting the directory.
>
> Having a D_CASEFOLD_LOOKUP avoids both issues, and seems to be a cheap
> solution to the problem.
>
> But, as explained above, due to the filename preserving semantics, we
> cannot just validate based on D_CASEFOLD_LOOKUP.
>
> For that, one solution would be to invalidate the negative dentry when
> it is decided to turn it positive, instead of reusing it. I implemented
> that in the past (2018) but Al Viro made it clear we don't want to incur
> costs on the VFS critical path for filesystems who don't care about
> case-insensitiveness.
>
> Instead, this patch invalidates negative dentries in casefold
> directories in d_revalidate during creation lookups, iff the lookup name
> is not exactly what is cached.  Other kinds of lookups wouldn't need
> this limitation.
>
> * caveats
>
> 1) Encryption
>
> Negative dentries on case-insensitive encrypted directories are also
> disabled.  No semantic change for them is intended in
> this patchset; we just bypass the revalidation directly to fscrypt, for
> positive dentries.  Encryption support is future work.
>
> 2) revalidate the cached dentry using the name under lookup
>
> Validating based on the lookup name is strange for a cache.  the new
> semantic is implemented by d_revalidate, to stay out of the critical
> path of filesystems who don't care about case-insensitiveness, as much
> as possible.  The only change is the addition of a new flavor of
> d_revalidate.
>
> * Tests
>
> There are a tests in place for most of the corner cases in generic/556.
> They mainly verify the name-preserving semantics.  The invalidation when
> converting the directory is harder to test, because it is hard to force
> the invalidation of specific cached dentries that occlude a dangling
> invalid dentry.  I tested it with forcing the positive dentries to be
> removed, but I'm not sure how to write an upstreamable test.
>
> It also survives fstests quick group regression testing on both ext4 and
> f2fs.
>
> * Performance
>
> The latency of lookups of non-existing files is obviously improved, as
> would be expected.  The following numbers compare the execution time of 10^6
> lookups of a non-existing file in a case-insensitive directory
> pre-populated with 100k files in ext4.
>
> Without the patch: 10.363s / 0.349s / 9.920s  (real/user/sys)
> With the patch:     1.752s / 0.276s / 1.472s  (real/user/sys)
>
> * patchset
>
> Patch 1 introduces a new flavor of d_revalidate to provide the
> filesystem with the name under lookup; Patch 2 introduces the new flag
> to signal the dentry creation context; Patch 3 introduces a libfs helper
> to revalidate negative dentries on case-insensitive directories; Patch 4
> deals with encryption; Patch 5 cleans up the now redundant dentry
> operations for case-insensitive with and without encryption; Finally,
> Patch 6 and 7 enable support on case-insensitive directories
> for ext4 and f2fs, respectively.
>
> Gabriel Krisman Bertazi (7):
>   fs: Expose name under lookup to d_revalidate hook
>   fs: Add DCACHE_CASEFOLD_LOOKUP flag
>   libfs: Validate negative dentries in case-insensitive directories
>   libfs: Support revalidation of encrypted case-insensitive dentries
>   libfs: Merge encrypted_ci_dentry_ops and ci_dentry_ops
>   ext4: Enable negative dentries on case-insensitive lookup
>   f2fs: Enable negative dentries on case-insensitive lookup
>
>  fs/dcache.c            | 10 +++++-
>  fs/ext4/namei.c        | 34 ++----------------
>  fs/f2fs/namei.c        | 23 ++----------
>  fs/libfs.c             | 82 ++++++++++++++++++++++++++----------------
>  fs/namei.c             | 23 +++++++-----
>  include/linux/dcache.h |  9 +++++
>  6 files changed, 88 insertions(+), 93 deletions(-)

-- 
Gabriel Krisman Bertazi

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