Re: [PATCH net-next v13 07/14] mm: page_frag: reuse existing space for 'size' and 'pfmemalloc'

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/8/15 23:03, Alexander Duyck wrote:
> On Wed, Aug 14, 2024 at 8:10 PM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>
>> On 2024/8/15 0:13, Alexander H Duyck wrote:
>>> On Thu, 2024-08-08 at 20:37 +0800, Yunsheng Lin wrote:
>>>> Currently there is one 'struct page_frag' for every 'struct
>>>> sock' and 'struct task_struct', we are about to replace the
>>>> 'struct page_frag' with 'struct page_frag_cache' for them.
>>>> Before begin the replacing, we need to ensure the size of
>>>> 'struct page_frag_cache' is not bigger than the size of
>>>> 'struct page_frag', as there may be tens of thousands of
>>>> 'struct sock' and 'struct task_struct' instances in the
>>>> system.
>>>>
>>>> By or'ing the page order & pfmemalloc with lower bits of
>>>> 'va' instead of using 'u16' or 'u32' for page size and 'u8'
>>>> for pfmemalloc, we are able to avoid 3 or 5 bytes space waste.
>>>> And page address & pfmemalloc & order is unchanged for the
>>>> same page in the same 'page_frag_cache' instance, it makes
>>>> sense to fit them together.
>>>>
>>>> After this patch, the size of 'struct page_frag_cache' should be
>>>> the same as the size of 'struct page_frag'.
>>>>
>>>> CC: Alexander Duyck <alexander.duyck@gmail.com>
>>>> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
>>>> ---
>>>>  include/linux/mm_types_task.h   | 16 +++++-----
>>>>  include/linux/page_frag_cache.h | 52 +++++++++++++++++++++++++++++++--
>>>>  mm/page_frag_cache.c            | 49 +++++++++++++++++--------------
>>>>  3 files changed, 85 insertions(+), 32 deletions(-)
>>>>
>>>> diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
>>>> index b1c54b2b9308..f2610112a642 100644
>>>> --- a/include/linux/mm_types_task.h
>>>> +++ b/include/linux/mm_types_task.h
>>>> @@ -50,18 +50,18 @@ struct page_frag {
>>>>  #define PAGE_FRAG_CACHE_MAX_SIZE    __ALIGN_MASK(32768, ~PAGE_MASK)
>>>>  #define PAGE_FRAG_CACHE_MAX_ORDER   get_order(PAGE_FRAG_CACHE_MAX_SIZE)
>>>>  struct page_frag_cache {
>>>> -    void *va;
>>>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
>>>> +    /* encoded_va consists of the virtual address, pfmemalloc bit and order
>>>> +     * of a page.
>>>> +     */
>>>> +    unsigned long encoded_va;
>>>> +
>>>
>>> Rather than calling this an "encoded_va" we might want to call this an
>>> "encoded_page" as that would be closer to what we are actually working
>>> with. We are just using the virtual address as the page pointer instead
>>> of the page struct itself since we need quicker access to the virtual
>>> address than we do the page struct.
>>
>> Calling it "encoded_page" seems confusing enough when calling virt_to_page()
>> with "encoded_page" when virt_to_page() is expecting a 'va', no?
> 
> It makes about as much sense as calling it an "encoded_va". What you
> have is essentially a packed page struct that contains the virtual
> address, pfmemalloc flag, and order. So if you want you could call it
> "packed_page" too I suppose. Basically this isn't a valid virtual
> address it is a page pointer with some extra metadata packed in.

I think we are all argeed that is not a valid virtual address by adding
the 'encoded_' part.
I am not really sure if "encoded_page" or "packed_page" is better than
'encoded_va' here, as there is no 'page pointer' that is implied by
"encoded_page" or "packed_page" here. For 'encoded_va', at least there
is 'virtual address' that is implied by 'encoded_va', and that 'virtual
address' just happen to be page pointer.

Yes, you may say the 'pfmemalloc flag and order' part is about page, not
about 'va', I guess there is trade-off we need to make here if there is
not a perfect name for it and 'va' does occupy most bits of 'encoded_va'.