Re: [PATCH v8 01/10] fs: Allow fine-grained control of folio sizes

[Ryan Roberts <ryan.roberts@arm.com>]

On 05/07/2024 14:24, Pankaj Raghav (Samsung) wrote:
>>> I suggest you handle it better than this.  If the device is asking for a
>>> blocksize > PMD_SIZE, you should fail to mount it.
>>
>> That's my point: we already do that.
>>
>> The largest block size we support is 64kB and that's way smaller
>> than PMD_SIZE on all platforms and we always check for bs > ps 
>> support at mount time when the filesystem bs > ps.
>>
>> Hence we're never going to set the min value to anything unsupported
>> unless someone makes a massive programming mistake. At which point,
>> we want a *hard, immediate fail* so the developer notices their
>> mistake immediately. All filesystems and block devices need to
>> behave this way so the limits should be encoded as asserts in the
>> function to trigger such behaviour.
> 
> I agree, this kind of bug will be encountered only during developement 
> and not during actual production due to the limit we have fs block size
> in XFS.
> 
>>
>>> If the device is
>>> asking for a blocksize > PAGE_SIZE and CONFIG_TRANSPARENT_HUGEPAGE is
>>> not set, you should also decline to mount the filesystem.
>>
>> What does CONFIG_TRANSPARENT_HUGEPAGE have to do with filesystems
>> being able to use large folios?
>>
>> If that's an actual dependency of using large folios, then we're at
>> the point where the mm side of large folios needs to be divorced
>> from CONFIG_TRANSPARENT_HUGEPAGE and always supported.
>> Alternatively, CONFIG_TRANSPARENT_HUGEPAGE needs to selected by the
>> block layer and also every filesystem that wants to support
>> sector/blocks sizes larger than PAGE_SIZE.  IOWs, large folio
>> support needs to *always* be enabled on systems that say
>> CONFIG_BLOCK=y.
> 
> Why CONFIG_BLOCK? I think it is enough if it comes from the FS side
> right? And for now, the only FS that needs that sort of bs > ps 
> guarantee is XFS with this series. Other filesystems such as bcachefs 
> that call mapping_set_large_folios() only enable it as an optimization
> and it is not needed for the filesystem to function.
> 
> So this is my conclusion from the conversation:
> - Add a dependency in Kconfig on THP for XFS until we fix the dependency
>   of large folios on THP

THP isn't supported on some arches, so isn't this effectively saying XFS can no
longer be used with those arches, even if the bs <= ps? I think while pagecache
large folios depend on THP, you need to make this a mount-time check in the FS?

But ideally, MAX_PAGECACHE_ORDER would be set to 0 for
!CONFIG_TRANSPARENT_HUGEPAGE so you can just check against that and don't have
to worry about THP availability directly.

Willy; Why is MAX_PAGECACHE_ORDER set to 8 when THP is disabled currently?

> - Add a BUILD_BUG_ON(XFS_MAX_BLOCKSIZE > MAX_PAGECACHE_ORDER)
> - Add a WARN_ON_ONCE() and clamp the min and max value in
>   mapping_set_folio_order_range() ?
> 
> Let me know what you all think @willy, @dave and @ryan.
> 
> --
> Pankaj