Re: [RFC PATCH 7/7] xfs: Use dax_is_supported()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-01-29 21:38, Dave Chinner wrote:
> On Mon, Jan 29, 2024 at 04:06:31PM -0500, Mathieu Desnoyers wrote:
>> Use dax_is_supported() to validate whether the architecture has
>> virtually aliased caches at mount time.
>>
>> This is relevant for architectures which require a dynamic check
>> to validate whether they have virtually aliased data caches
>> (ARCH_HAS_CACHE_ALIASING_DYNAMIC=y).
> 
> Where's the rest of this patchset? I have no idea what
> dax_is_supported() actually does, how it interacts with
> CONFIG_FS_DAX, etc.
> 
> If you are changing anything to do with FSDAX, the cc-ing the
> -entire- patchset to linux-fsdevel is absolutely necessary so the
> entire patchset lands in our inboxes and not just a random patch
> from the middle of a bigger change.

Sorry, I will Cc linux-fsdevel on all patches for the next round.

Meanwhile you can find the whole series on lore:

https://lore.kernel.org/lkml/20240129210631.193493-1-mathieu.desnoyers@efficios.com/

[...]

> 
> Assuming that I understand what dax_is_supported() is doing, this
> change isn't right.  We're just setting the DAX configuration flags
> from the mount options here, we don't validate them until
> we've parsed all options and eliminated conflicts and rejected
> conflicting options. We validate whether the options are
> appropriate for the underlying hardware configuration later in the
> mount process.
> 
> dax=always suitability is check in xfs_setup_dax_always() called
> later in the mount process when we have enough context and support
> to open storage devices and check them for DAX support. If the
> hardware does not support DAX then we simply we turn off DAX
> support, we do not reject the mount as this change does.
> 
> dax=inode and dax=never are valid options on all configurations,
> even those with without FSDAX support or have hardware that is not
> capable of using DAX. dax=inode only affects how an inode is
> instantiated in cache - if the inode has a flag that says "use DAX"
> and dax is suppoortable by the hardware, then the turn on DAX for
> that inode. Otherwise we just use the normal non-dax IO paths.
> 
> Again, we don't error out the filesystem if DAX is not supported,
> we just don't turn it on. This check is done in
> xfs_inode_should_enable_dax() and I think all you need to do is
> replace the IS_ENABLED(CONFIG_FS_DAX) with a dax_is_supported()
> call...

Thanks a lot for the detailed explanation. You are right, I will
move the dax_is_supported() check to xfs_inode_should_enable_dax().

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com