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
