Re: [RFC PATCH v2 1/2] mm: shmem: add large folio support to the write and fallocate paths

[Daniel Gomez <da.gomez@samsung.com>]

On 9/26/2024 2:16 PM, Matthew Wilcox wrote:
> On Thu, Sep 26, 2024 at 04:27:26PM +0800, Baolin Wang wrote:
>> +static inline unsigned int
>> +shmem_mapping_size_order(struct address_space *mapping, pgoff_t index, size_t size)
>> +{
>> +	unsigned int order = get_order(max_t(size_t, size, PAGE_SIZE));
> 
> Why introduce the max_t() call here?  Did nobody read the documentation
> or implementation for get_order() before writing this patch?

get_order() result is undefined if the size is 0. I've used max_t() here 
to avoid that case. Perhaps should we prevent that case before getting here?

> 
> Besides, get_order() is wrong (at least relative to other filesystems).
> get_order() rounds up instead of down, so what should we do for a write()
> of size 512 * 1024 + 1 byte?  Other filesystems allocate an order-8 folio
> plus an order-0 folio.  This code would have us allocate an order-9 folio.
> I think that's a bad idea.
> 

I think one of my earlier attemps was to use fgf_set_order + 
FGF_GET_ORDER() as in iomap. But the solution taken there was to share 
code between shmem and filemap and that wasn't considered a good idea. 
Shall we just replicate iomap_get_folio()? Or else, what do you suggest 
here?

Daniel