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

[Matthew Wilcox]

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?

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.