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

From: Daniel Gomez
Date: Thu Sep 26 2024 - 08:58:51 EST


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