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

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Message-ID: <CAKgT0UeUhf_FuGRuEBkrqNSGWd3a4FbY=FLhqbdcM+BhdFGjGQ@mail.gmail.com>
Date: Mon, 19 Aug 2024 09:00:02 -0700
From: Alexander Duyck <alexander.duyck@...il.com>
To: Yunsheng Lin <linyunsheng@...wei.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH net-next v13 07/14] mm: page_frag: reuse existing space
 for 'size' and 'pfmemalloc'

On Fri, Aug 16, 2024 at 4:56 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2024/8/15 23:03, Alexander Duyck wrote:
> > On Wed, Aug 14, 2024 at 8:10 PM Yunsheng Lin <linyunsheng@...wei.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@...il.com>
> >>>> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.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.

Basically we are using the page's virtual address to encode the page
into the struct. If you look, "virtual" is a pointer stored in the
page to provide the virtual address on some architectures. It also
happens that we have virt_to_page which provides an easy way to get
back and forth between the values.

> 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'.

The naming isn't really a show stopper one way or another. It was more
the fact that you had several functions accessing it that were using
the name "encoded_page" as I recall. That is why I thought it might
make sense to rename it to that. Why have functions called
"encoded_page_order" work with an "encoded_va" versus an
"encoded_page". It makes it easier to logically lump them all
together.