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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/9/26 21:40, Matthew Wilcox wrote:
> On Thu, Sep 26, 2024 at 02:58:31PM +0200, Daniel Gomez wrote:
>> 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?
> 
> Surely we've handled a length-0 write before we get here?
> 
>> 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?
> 
> We could move three of the four lines from fgf_set_order() into a
> new function and call it from both fgf_set_order() and shmem?

Sounds good. How about the following changes? Do you have a perferred 
name for the new helper? Thanks.

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index d9c7edb6422b..ce418acd2737 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -629,6 +629,16 @@ typedef unsigned int __bitwise fgf_t;

  #define FGP_WRITEBEGIN         (FGP_LOCK | FGP_WRITE | FGP_CREAT | 
FGP_STABLE)

+static inline unsigned int filemap_get_order(size_t size)
+{
+       unsigned int shift = ilog2(size);
+
+       if (shift <= PAGE_SHIFT)
+               return 0;
+
+       return shift - PAGE_SHIFT;
+}
+
  /**
   * fgf_set_order - Encode a length in the fgf_t flags.
   * @size: The suggested size of the folio to create.
@@ -642,11 +652,11 @@ typedef unsigned int __bitwise fgf_t;
   */
  static inline fgf_t fgf_set_order(size_t size)
  {
-       unsigned int shift = ilog2(size);
+       unsigned int order = filemap_get_order(size);

-       if (shift <= PAGE_SHIFT)
+       if (!order)
                 return 0;
-       return (__force fgf_t)((shift - PAGE_SHIFT) << 26);
+       return (__force fgf_t)(order << 26);
  }

  void *filemap_get_entry(struct address_space *mapping, pgoff_t index);