linux-ext4 - Re: [f2fs-dev] [PATCH v6 0/9] Support negative dentries on case-insensitive ext4 and f2fs

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Message-ID: <CAHk-=wgADgC_+Nmamrzei7JpRDa7ugvP8P_8zS2VxB5ksF9Khg@mail.gmail.com>
Date: Thu, 23 Nov 2023 08:41:37 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Gabriel Krisman Bertazi <gabriel@...sman.be>
Cc: Al Viro <viro@...iv.linux.org.uk>, Christian Brauner <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 v6 0/9] Support negative dentries on
 case-insensitive ext4 and f2fs

On Thu, 23 Nov 2023 at 07:57, Gabriel Krisman Bertazi
<gabriel@...sman.be> wrote:
>
> The problem I found with that approach, which I originally tried, was
> preventing concurrent lookups from racing with the invalidation and
> creating more 'case-sensitive' negative dentries.  Did I miss a way to
> synchronize with concurrent lookups of the children of the dentry?  We
> can trivially ensure the dentry doesn't have positive children by
> holding the parent lock, but that doesn't protect from concurrent
> lookups creating negative dentries, as far as I understand.

I'd just set the "casefolded" bit, then do a RCU grace period wait,
and then invalidate all old negative dentries.

Sure, there's technically a window there where somebody could hit an
existing negative dentry that matches a casefolded name after
casefolded has been set (but before the invalidation) and the lookup
would result in a "does not exist" lookup that way.

But that seems no different from the lookup having been done before
the casefolded bit got set, so I don't think that's an _actual_
difference. If you do a lookup concurrently with the directory being
set casefolded, you get one or the other.

And no, I haven't thought about this a ton, but it seems the obvious
thing to do. Temporary stale negative dentries while the casefolded
bit is in the process of being set seems a harmless thing, exactly
because they would seem to be the same thing as if the lookup was done
before...

And yes, that "wait for RCU grace period" is a somewhat slow
operation, but how often do the casefolded bits get changed?

This is not a huge deal. I don't hate your approach, I just found it surprising.

                 Linus