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:

quoted

On Thu, Jan 7, 2021 at 5:27 PM Theodore Ts'o [off-list ref] wrote:

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On Thu, Jan 07, 2021 at 01:37:47PM +0000, Russell King - ARM Linux admin wrote:

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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