Re: [RFC v11 02/14] mm: move the page fragment allocator from page_alloc into its own file

[Alexander Duyck <alexander.duyck@gmail.com>]

On Fri, Jul 19, 2024 at 2:37 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> Inspired by [1], move the page fragment allocator from page_alloc
> into its own c file and header file, as we are about to make more
> change for it to replace another page_frag implementation in
> sock.c
>
> As this patchset is going to replace 'struct page_frag' with
> 'struct page_frag_cache' in sched.h, including page_frag_cache.h
> in sched.h has a compiler error caused by interdependence between
> mm_types.h and mm.h for asm-offsets.c, see [2]. So avoid the compiler
> error by moving 'struct page_frag_cache' to mm_types_task.h as
> suggested by Alexander, see [3].
>
> 1. https://lore.kernel.org/all/20230411160902.4134381-3-dhowells@redhat.com/
> 2. https://lore.kernel.org/all/15623dac-9358-4597-b3ee-3694a5956920@gmail.com/
> 3. https://lore.kernel.org/all/CAKgT0UdH1yD=LSCXFJ=YM_aiA4OomD-2wXykO42bizaWMt_HOA@mail.gmail.com/
> CC: David Howells <dhowells@redhat.com>
> CC: Alexander Duyck <alexander.duyck@gmail.com>
> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
> ---
>  include/linux/gfp.h             |  22 -----
>  include/linux/mm_types.h        |  18 ----
>  include/linux/mm_types_task.h   |  18 ++++
>  include/linux/page_frag_cache.h |  32 +++++++
>  include/linux/skbuff.h          |   1 +
>  mm/Makefile                     |   1 +
>  mm/page_alloc.c                 | 136 ------------------------------
>  mm/page_frag_cache.c            | 145 ++++++++++++++++++++++++++++++++
>  mm/page_frag_test.c             |   2 +-
>  9 files changed, 198 insertions(+), 177 deletions(-)
>  create mode 100644 include/linux/page_frag_cache.h
>  create mode 100644 mm/page_frag_cache.c
>

...

> diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
> index a2f6179b672b..cdc1e3696439 100644
> --- a/include/linux/mm_types_task.h
> +++ b/include/linux/mm_types_task.h
> @@ -8,6 +8,7 @@
>   * (These are defined separately to decouple sched.h from mm_types.h as much as possible.)
>   */
>
> +#include <linux/align.h>
>  #include <linux/types.h>
>
>  #include <asm/page.h>
> @@ -46,6 +47,23 @@ struct page_frag {
>  #endif
>  };
>
> +#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)
> +       __u16 offset;
> +       __u16 size;
> +#else
> +       __u32 offset;
> +#endif
> +       /* we maintain a pagecount bias, so that we dont dirty cache line
> +        * containing page->_refcount every time we allocate a fragment.
> +        */
> +       unsigned int            pagecnt_bias;
> +       bool pfmemalloc;
> +};
> +
>  /* Track pages that require TLB flushes */
>  struct tlbflush_unmap_batch {
>  #ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h
> new file mode 100644
> index 000000000000..43afb1bbcac9
> --- /dev/null
> +++ b/include/linux/page_frag_cache.h
> @@ -0,0 +1,32 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_PAGE_FRAG_CACHE_H
> +#define _LINUX_PAGE_FRAG_CACHE_H
> +
> +#include <linux/log2.h>
> +#include <linux/types.h>
> +#include <linux/mm_types_task.h>

You don't need to include mm_types_task.h here. You can just use
declare "struct page_frag_cache;" as we did before in gfp.h.
Technically this should be included in mm_types.h so any callers
making use of these functions would need to make sure to include that
like we did for gfp.h before anyway.

> +#include <asm/page.h>
> +

Not sure why this is included here either. From what I can tell there
isn't anything here using the contents of page.h. I suspect you should
only need it for the get_order call which would be used in other
files.

> +void page_frag_cache_drain(struct page_frag_cache *nc);
> +void __page_frag_cache_drain(struct page *page, unsigned int count);
> +void *__page_frag_alloc_align(struct page_frag_cache *nc, unsigned int fragsz,
> +                             gfp_t gfp_mask, unsigned int align_mask);
> +
> +static inline void *page_frag_alloc_align(struct page_frag_cache *nc,
> +                                         unsigned int fragsz, gfp_t gfp_mask,
> +                                         unsigned int align)
> +{
> +       WARN_ON_ONCE(!is_power_of_2(align));
> +       return __page_frag_alloc_align(nc, fragsz, gfp_mask, -align);
> +}
> +
> +static inline void *page_frag_alloc(struct page_frag_cache *nc,
> +                                   unsigned int fragsz, gfp_t gfp_mask)
> +{
> +       return __page_frag_alloc_align(nc, fragsz, gfp_mask, ~0u);
> +}
> +
> +void page_frag_free(void *addr);
> +
> +#endif

...

> diff --git a/mm/page_frag_test.c b/mm/page_frag_test.c
> index cf2691f60b67..b7a5affb92f2 100644
> --- a/mm/page_frag_test.c
> +++ b/mm/page_frag_test.c
> @@ -6,7 +6,6 @@
>   * Copyright: linyunsheng@huawei.com
>   */
>
> -#include <linux/mm.h>
>  #include <linux/module.h>
>  #include <linux/slab.h>
>  #include <linux/vmalloc.h>
> @@ -16,6 +15,7 @@
>  #include <linux/log2.h>
>  #include <linux/completion.h>
>  #include <linux/kthread.h>
> +#include <linux/page_frag_cache.h>
>
>  #define OBJPOOL_NR_OBJECT_MAX  BIT(24)

Rather than making users have to include page_frag_cache.h I think it
would be better for us to just maintain the code as being accessible
from mm.h. So it might be better to just add page_frag_cache.h to the
includes there.