Re: MAP_UNINITIALIZED (Was Re: MAP_NOZERO revisited)

[Balbir Singh <bsingharora@gmail.com>]

> commit 37b83f3fb77a177a2f81ebb8aeaec28c2a46e503
> Author: Arun Sharma <asharma@fb.com>
> Date:   Tue Jan 10 17:02:46 2012 -0800
>
>    mm: Enable MAP_UNINITIALIZED for archs with mmu
>
>    This enables malloc optimizations where we might
>    madvise(..,MADV_DONTNEED) a page only to fault it
>    back at a different virtual address.
>
>    Signed-off-by: Arun Sharma <asharma@fb.com>
>
> diff --git a/include/asm-generic/mman-common.h b/include/asm-generic/mman-common.h
> index 787abbb..71e079f 100644
> --- a/include/asm-generic/mman-common.h
> +++ b/include/asm-generic/mman-common.h
> @@ -19,11 +19,7 @@
>  #define MAP_TYPE       0x0f            /* Mask for type of mapping */
>  #define MAP_FIXED      0x10            /* Interpret addr exactly */
>  #define MAP_ANONYMOUS  0x20            /* don't use a file */
> -#ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
> -# define MAP_UNINITIALIZED 0x4000000   /* For anonymous mmap, memory could be uninitialized */
> -#else
> -# define MAP_UNINITIALIZED 0x0         /* Don't support this flag */
> -#endif
> +#define MAP_UNINITIALIZED 0x4000000    /* For anonymous mmap, memory could be uninitialized */
>

Define MAP_UNINITIALIZED - are you referring to not zeroing out pages
before handing them down? Is this safe even between threads.

>  #define MS_ASYNC       1               /* sync memory asynchronously */
>  #define MS_INVALIDATE  2               /* invalidate the caches */
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 3a93f73..04d838e 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -156,6 +156,11 @@ __alloc_zeroed_user_highpage(gfp_t movableflags,
>        struct page *page = alloc_page_vma(GFP_HIGHUSER | movableflags,
>                        vma, vaddr);
>
> +#ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
> +       if (!vma->vm_file && vma->vm_flags & VM_UNINITIALIZED)
> +               return page;
> +#endif
> +
>        if (page)
>                clear_user_highpage(page, vaddr);
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4baadd1..6345c57 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -118,6 +118,8 @@ extern unsigned int kobjsize(const void *objp);
>  #define VM_SAO         0x20000000      /* Strong Access Ordering (powerpc) */
>  #define VM_PFN_AT_MMAP 0x40000000      /* PFNMAP vma that is fully mapped at mmap time */
>  #define VM_MERGEABLE   0x80000000      /* KSM may merge identical pages */
> +#define VM_UNINITIALIZED VM_SAO                /* Steal a powerpc bit for now, since we're out
> +                                          bits for 32 bit archs */

Without proper checks if it can be re-used?

>
>  /* Bits set in the VMA until the stack is in its final location */
>  #define VM_STACK_INCOMPLETE_SETUP      (VM_RAND_READ | VM_SEQ_READ)
> diff --git a/include/linux/mman.h b/include/linux/mman.h
> index 51647b4..f7d4f60 100644
> --- a/include/linux/mman.h
> +++ b/include/linux/mman.h
> @@ -88,6 +88,7 @@ calc_vm_flag_bits(unsigned long flags)
>        return _calc_vm_trans(flags, MAP_GROWSDOWN,  VM_GROWSDOWN ) |
>               _calc_vm_trans(flags, MAP_DENYWRITE,  VM_DENYWRITE ) |
>               _calc_vm_trans(flags, MAP_EXECUTABLE, VM_EXECUTABLE) |
> +              _calc_vm_trans(flags, MAP_UNINITIALIZED, VM_UNINITIALIZED) |
>               _calc_vm_trans(flags, MAP_LOCKED,     VM_LOCKED    );
>  }
>  #endif /* __KERNEL__ */
> diff --git a/init/Kconfig b/init/Kconfig
> index 43298f9..428e047 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1259,7 +1259,7 @@ endchoice
>
>  config MMAP_ALLOW_UNINITIALIZED
>        bool "Allow mmapped anonymous memory to be uninitialized"
> -       depends on EXPERT && !MMU
> +       depends on EXPERT
>        default n
>        help
>          Normally, and according to the Linux spec, anonymous memory obtained
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index c3fdbcb..e6dd642 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1868,6 +1868,12 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
>                put_mems_allowed();
>                return page;
>        }
> +
> +#ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
> +       if (!vma->vm_file && vma->vm_flags & VM_UNINITIALIZED)
> +               gfp &= ~__GFP_ZERO;
> +#endif
> +
>        /*
>         * fast path:  default or task policy
>         */

Balbir

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