linux-ext4 - Re: Aarch64 EXT4FS inode checksum failures - seems to be weak memory ordering issues

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Date:   Thu, 7 Jan 2021 15:53:28 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Arnd Bergmann <arnd@...nel.org>, "Theodore Ts'o" <tytso@....edu>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>,
        linux-toolchains@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Aarch64 EXT4FS inode checksum failures - seems to be weak memory
 ordering issues

On Thu, Jan 07, 2021 at 02:27:51PM -0800, Eric Biggers wrote:
> On Thu, Jan 07, 2021 at 10:48:05PM +0100, Arnd Bergmann wrote:
> > On Thu, Jan 7, 2021 at 5:27 PM Theodore Ts'o <tytso@....edu> wrote:
> > >
> > > On Thu, Jan 07, 2021 at 01:37:47PM +0000, Russell King - ARM Linux admin wrote:
> > > > > The gcc bugzilla mentions backports into gcc-linaro, but I do not see
> > > > > them in my git history.
> > > >
> > > > So, do we raise the minimum gcc version for the kernel as a whole to 5.1
> > > > or just for aarch64?
> > >
> > > Russell, Arnd, thanks so much for tracking down the root cause of the
> > > bug!
> > 
> > There is one more thing that I wondered about when looking through
> > the ext4 code: Should it just call the crc32c_le() function directly
> > instead of going through the crypto layer? It seems that with Ard's
> > rework from 2018, that can just call the underlying architecture specific
> > implementation anyway.
> > 
> 
> It looks like that would work, although note that crc32c_le() uses the shash API
> too, so it isn't any more "direct" than what ext4 does now.

Yes.

> Also, a potential issue is that the implementation of crc32c that crc32c_le()
> uses might be chosen too early if the architecture-specific implementation of
> crc32c is compiled as a module (e.g. crc32c-intel.ko).

This was the primary reason I chose to do it this way for ext4.

The other is that ext4 didn't use crc32c before metadata_csum, so
there's no point in pulling in the crypto layer if you're only going to
use older ext2 or ext3 filesystems.  That was 2010, maybe people have
stopped doing that?

> There are two ways this
> could be fixed -- either by making it a proper library API like blake2s() that
> can call the architecture-specific code directly, or by reconfiguring things
> when a new crypto module is loaded (like what lib/crc-t10dif.c does).

Though I would like to see the library functions gain the ability to use
whatever is the fastest mechanism available once we can be reasonably
certain that all the platform-specific drivers have been loaded.

That said, IIRC most distros compile all of them into their
(increasingly large) vmlinuz files so maybe this isn't much of practical
concern?

--D
> 
> Until one of those is done, switching to crc32c_le() might cause performance
> regressions.
> 
> - Eric