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From: Al Viro <>
To: "André Almeida" <>
Cc: Hugh Dickins <>,
	Andrew Morton <>,,,,,,,
	Daniel Rosenberg <>
Subject: Re: [RFC PATCH 2/4] mm: shmem: Support case-insensitive file name lookups
Date: Tue, 23 Mar 2021 23:19:00 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, Mar 23, 2021 at 04:59:39PM -0300, André Almeida wrote:

> * dcache handling:
> For a +F directory, tmpfs only stores the first equivalent name dentry
> used in the dcache. This is done to prevent unintentional duplication of
> dentries in the dcache, while also allowing the VFS code to quickly find
> the right entry in the cache despite which equivalent string was used in
> a previous lookup, without having to resort to ->lookup().
> d_hash() of casefolded directories is implemented as the hash of the
> casefolded string, such that we always have a well-known bucket for all
> the equivalencies of the same string. d_compare() uses the
> utf8_strncasecmp() infrastructure, which handles the comparison of
> equivalent, same case, names as well.
> For now, negative lookups are not inserted in the dcache, since they
> would need to be invalidated anyway, because we can't trust missing file
> dentries. This is bad for performance but requires some leveraging of
> the VFS layer to fix. We can live without that for now, and so does
> everyone else.

"For now"?  Not a single practical suggestion has ever materialized.
Pardon me, but by now I'm very sceptical about the odds of that
ever changing.  And no, I don't have any suggestions either.

> The lookup() path at tmpfs creates negatives dentries, that are later
> instantiated if the file is created. In that way, all files in tmpfs
> have a dentry given that the filesystem exists exclusively in memory.
> As explained above, we don't have negative dentries for casefold files,
> so dentries are created at lookup() iff files aren't casefolded. Else,
> the dentry is created just before being instantiated at create path.
> At the remove path, dentries are invalidated for casefolded files.

Umm...  What happens to those assertions if previously sane directory
gets case-buggered?  You've got an ioctl for doing just that...
Incidentally, that ioctl is obviously racy - result of that simple_empty() 
might have nothing to do with reality before it is returned to caller.
And while we are at it, simple_empty() doesn't check a damn thing about
negative dentries in there...

  parent reply	other threads:[~2021-03-23 23:21 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-23 19:59 [RFC PATCH 0/4] mm: shmem: Add case-insensitive support for tmpfs André Almeida
2021-03-23 19:59 ` [RFC PATCH 1/4] Revert "libfs: unexport generic_ci_d_compare() and generic_ci_d_hash()" André Almeida
2021-03-23 20:15   ` Matthew Wilcox
2021-03-24 20:09     ` André Almeida
2021-03-23 19:59 ` [RFC PATCH 2/4] mm: shmem: Support case-insensitive file name lookups André Almeida
2021-03-23 20:18   ` Gabriel Krisman Bertazi
2021-03-24 20:17     ` André Almeida
2021-03-23 23:19   ` Al Viro [this message]
2021-03-24 20:44     ` André Almeida
2021-03-23 19:59 ` [RFC PATCH 3/4] mm: shmem: Add IOCTL support for tmpfs André Almeida
2021-03-23 22:15   ` Gabriel Krisman Bertazi
2021-03-23 19:59 ` [RFC PATCH 4/4] docs: tmpfs: Add casefold options André Almeida
2021-03-23 21:58   ` Randy Dunlap
2021-03-25 14:27     ` André Almeida
2021-03-23 22:19   ` Gabriel Krisman Bertazi
2021-03-24 20:47     ` André Almeida

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