Re: [PATCH -v2] ext4: introduce linear search for dentries

Andreas Dilger
2025-02-20T01:58:08Z

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On Feb 19, 2025, at 2:43 PM, Gabriel Krisman Bertazi <krisman@suse.de> wrote:
>
> Andreas Dilger <adilger@dilger.ca> writes:
>
>> On Feb 13, 2025, at 1:10 PM, Theodore Ts'o <tytso@mit.edu> wrote:
>>>
>>> This patch addresses an issue where some files in case-insensitive
>>> directories become inaccessible due to changes in how the kernel
>>> function, utf8_casefold(), generates case-folded strings from the
>>> commit 5c26d2f1d3f5 ("unicode: Don't special case ignorable code
>>> points").
>>>
>>> There are good reasons why this change should be made; it's actually
>>> quite stupid that Unicode seems to think that the characters ❤ and ❤️
>>> should be casefolded. Unfortimately because of the backwards
>>> compatibility issue, this commit was reverted in 231825b2e1ff.
>>>
>>> This problem is addressed by instituting a brute-force linear fallback
>>> if a lookup fails on case-folded directory, which does result in a
>>> performance hit when looking up files affected by the changing how
>>> thekernel treats ignorable Uniode characters, or when attempting to
>>> look up non-existent file names. So this fallback can be disabled by
>>> setting an encoding flag if in the future, the system administrator or
>>> the manufacturer of a mobile handset or tablet can be sure that there
>>> was no opportunity for a kernel to insert file names with incompatible
>>> encodings.
>>
>> I don't have the full context here, but falling back to a full directory
>> scan for every failed lookup in a casefolded directory would be *very*
>> expensive for a large directory.
>
> The context is that I made a change in unicode that caused a change in
> the filename hash of case-insensitive dirents, which are calculated from
> the casefolded form of the name. While that change was reverted, users
> have created files with the broken kernels and there are reports of
> files inaccessible.
>
>> This could be made conditional upon a much narrower set of conditions:
>> - if the filename has non-ASCII characters (already uncommon)
>> - if the filename contains characters that may be case folded
>> (normalized?)
>
> It could be even simpler, by only doing it to filenames that have
> zero-length characters before normalization. We can easily check it
> with utf8nlen or utf8ncursor. I'm very wary of differentiating ASCII
> from other characters if we can avoid it.

Sure, my suggestions are aimed at minimizing the impact of this extra
(and very expensive) fallback mechanism. If there is a direct way to
determine which filenames were impacted by the earlier bug, and then
do only two lookups (one with the "buggy" casefolded name, one with the
"good" casefolded name) then this would be (at worst) a 2x slowdown for
the lookup, instead of a 1000x slowdown (or whatever, for large directories).

Also, since the number of users affected by this bug is relatively small
(only users running kernels >= v6.12-rc2-1-g5c26d2f1d3f5 where the broken
patch landed and v6.13-rc2-36-g231825b2e1ff when it was reverted), but the
workaround by default affects everyone using the casefold feature, it
behooves us to minimize the performance impact of the workaround.

>
>> This avoids a huge performance hit for every name lookup in very common
>> workloads that do not need it (i.e. most computer-generated filenames are
>> still only using ASCII characters).
>
> Right. But this also only affects case-insensitive filesystems, which
> have very specific and controlled applications and hardly thousands of
> files in the same directory.

I think this is a generalization that does not hold true in all cases.

We have been looking at adding casefold support to Lustre, in order to
improve Samba export performance (which also has a "scan all entries"
fallback), and we cannot control how many files are in a single directory.

It seems likely that systems have been using casefold directly on ext4
for Samba as well. If the performance impact of "scan all entries" is
noticeable for Samba, then it would be noticeable for this fallback.

> If we really need it, I'd suggest we don't differentiating ASCII from
> utf8, but only filenames with those sequences.

Even better if the fallback can be limited to the subset of affected names.

> IMO, Ted's patch seems fine as a temporary stopgap to recover those
> filesystems.

Sure, but it is enabled by default and affects anyone using the casefold
feature forever into the future, unless the plan is not to land this patch
and only deploy it in the specific distros where the broken kernel was used?

One option would be to have the kernel re-hash any entries that it finds
with the old filename, so that the directories repair themselves, and the
workaround could be removed after some time. Also, have e2fsck re-hash
the filenames in this case, so that there is a long-term solution after
the kernel workaround is removed.

Cheers, Andreas

>> Also, depending on the size of the directory vs. the number of case-folded
>> (normalized?) characters in the filename, it might be faster to do
>> 2^(ambiguous_chars) htree lookups instead of a linear scan of the whole dir.
>>
>> That could be checked easily whether 2^(ambiguous_chars) < dir blocks, since
>> the htree leaf blocks will always be fully scanned anyway once found. That
>> could be a big win if there are only a few remapped characters in a filename.
>>
>> Cheers, Andreas
>>
>>>
>>> Fixes: 5c26d2f1d3f5 ("unicode: Don't special case ignorable code points")
>>> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
>>> Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
>>> ---
>>> v2:
>>> * Fix compile failure when CONFIG_UNICODE is not enabled
>>> * Added reviewed-by from Gabriel Krisman
>>>
>>> fs/ext4/namei.c | 14 ++++++++++----
>>> include/linux/fs.h | 10 +++++++++-
>>> 2 files changed, 19 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
>>> index 536d56d15072..820e7ab7f3a3 100644
>>> --- a/fs/ext4/namei.c
>>> +++ b/fs/ext4/namei.c
>>> @@ -1462,7 +1462,8 @@ static bool ext4_match(struct inode *parent,
>>> * sure cf_name was properly initialized before
>>> * considering the calculated hash.
>>> */
>>> - if (IS_ENCRYPTED(parent) && fname->cf_name.name &&
>>> + if (sb_no_casefold_compat_fallback(parent->i_sb) &&
>>> + IS_ENCRYPTED(parent) && fname->cf_name.name &&
>>> (fname->hinfo.hash != EXT4_DIRENT_HASH(de) ||
>>> fname->hinfo.minor_hash != EXT4_DIRENT_MINOR_HASH(de)))
>>> return false;
>>> @@ -1595,10 +1596,15 @@ static struct buffer_head *__ext4_find_entry(struct inode *dir,
>>> * return. Otherwise, fall back to doing a search the
>>> * old fashioned way.
>>> */
>>> - if (!IS_ERR(ret) || PTR_ERR(ret) != ERR_BAD_DX_DIR)
>>> + if (IS_ERR(ret) && PTR_ERR(ret) == ERR_BAD_DX_DIR)
>>> + dxtrace(printk(KERN_DEBUG "ext4_find_entry: dx failed, "
>>> + "falling back\n"));
>>> + else if (!sb_no_casefold_compat_fallback(dir->i_sb) &&
>>> + *res_dir == NULL && IS_CASEFOLDED(dir))
>>> + dxtrace(printk(KERN_DEBUG "ext4_find_entry: casefold "
>>> + "failed, falling back\n"));
>>> + else
>>> goto cleanup_and_exit;
>>> - dxtrace(printk(KERN_DEBUG "ext4_find_entry: dx failed, "
>>> - "falling back\n"));
>>> ret = NULL;
>>> }
>>> nblocks = dir->i_size >> EXT4_BLOCK_SIZE_BITS(sb);
>>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>>> index 2c3b2f8a621f..aa4ec39202c3 100644
>>> --- a/include/linux/fs.h
>>> +++ b/include/linux/fs.h
>>> @@ -1258,11 +1258,19 @@ extern int send_sigurg(struct file *file);
>>> #define SB_NOUSER BIT(31)
>>>
>>> /* These flags relate to encoding and casefolding */
>>> -#define SB_ENC_STRICT_MODE_FL (1 << 0)
>>> +#define SB_ENC_STRICT_MODE_FL (1 << 0)
>>> +#define SB_ENC_NO_COMPAT_FALLBACK_FL (1 << 1)
>>>
>>> #define sb_has_strict_encoding(sb) \
>>> (sb->s_encoding_flags & SB_ENC_STRICT_MODE_FL)
>>>
>>> +#if IS_ENABLED(CONFIG_UNICODE)
>>> +#define sb_no_casefold_compat_fallback(sb) \
>>> + (sb->s_encoding_flags & SB_ENC_NO_COMPAT_FALLBACK_FL)
>>> +#else
>>> +#define sb_no_casefold_compat_fallback(sb) (1)
>>> +#endif
>>> +
>>> /*
>>> * Umount options
>>> */
>>> --
>>> 2.45.2
>>>
>>>
>>
>>
>> Cheers, Andreas
>>
>>
>>
>>
>>
>
> --
> Gabriel Krisman Bertazi

Cheers, Andreas

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