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

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

On 04/07/2024 16:20, Matthew Wilcox wrote:
> On Thu, Jul 04, 2024 at 01:23:20PM +0100, Ryan Roberts wrote:
>>> -	AS_LARGE_FOLIO_SUPPORT = 6,
>>
>> nit: this removed enum is still referenced in a comment further down the file.
> 
> Thanks.  Pankaj, let me know if you want me to send you a patch or if
> you'll do it directly.
> 
>>> +	/* Bits 16-25 are used for FOLIO_ORDER */
>>> +	AS_FOLIO_ORDER_BITS = 5,
>>> +	AS_FOLIO_ORDER_MIN = 16,
>>> +	AS_FOLIO_ORDER_MAX = AS_FOLIO_ORDER_MIN + AS_FOLIO_ORDER_BITS,
>>
>> nit: These 3 new enums seem a bit odd.
> 
> Yes, this is "too many helpful suggestions" syndrome.  It made a lot
> more sense originally.

Well now you can add my "helpful" suggestion to that list :)

> 
> https://lore.kernel.org/linux-fsdevel/ZlUQcEaP3FDXpCge@dread.disaster.area/
> 
>>> +static inline void mapping_set_folio_order_range(struct address_space *mapping,
>>> +						 unsigned int min,
>>> +						 unsigned int max)
>>> +{
>>> +	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
>>> +		return;
>>> +
>>> +	if (min > MAX_PAGECACHE_ORDER)
>>> +		min = MAX_PAGECACHE_ORDER;
>>> +	if (max > MAX_PAGECACHE_ORDER)
>>> +		max = MAX_PAGECACHE_ORDER;
>>> +	if (max < min)
>>> +		max = min;
>>
>> It seems strange to silently clamp these? Presumably for the bs>ps usecase,
>> whatever values are passed in are a hard requirement? So wouldn't want them to
>> be silently reduced. (Especially given the recent change to reduce the size of
>> MAX_PAGECACHE_ORDER to less then PMD size in some cases).
> 
> Hm, yes.  We should probably make this return an errno.  Including
> returning an errno for !IS_ENABLED() and min > 0.

Right.

> 
>>> -	if (new_order < MAX_PAGECACHE_ORDER) {
>>> +	if (new_order < mapping_max_folio_order(mapping)) {
>>>  		new_order += 2;
>>> -		new_order = min_t(unsigned int, MAX_PAGECACHE_ORDER, new_order);
>>> +		new_order = min(mapping_max_folio_order(mapping), new_order);
>>>  		new_order = min_t(unsigned int, new_order, ilog2(ra->size));
>>
>> I wonder if its possible that ra->size could ever be less than
>> mapping_min_folio_order()? Do you need to handle that?
> 
> I think we take care of that in later patches? 

Yes I saw that once I got to it. You can ignore this.

> This patch is mostly
> about honouring the max properly and putting in the infrastructure for
> the min, but not doing all the necessary work for min.