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SubjectRe: [PATCH v3] NOMMU: fix malloc performance by adding uninitialized flag

Mike Frysinger wrote:
> From: Jie Zhang <jie.zhang@analog.com>
>
> The NOMMU code currently clears all anonymous mmapped memory. While this
> is what we want in the default case, all memory allocation from userspace
> under NOMMU has to go through this interface, including malloc() which is
> allowed to return uninitialized memory. This can easily be a significant
> performance penalty. So for constrained embedded systems were security is
> irrelevant, allow people to avoid clearing memory unnecessarily.
>
> This also alters the ELF-FDPIC binfmt such that it obtains uninitialised
> memory for the brk and stack region.
>
> Signed-off-by: Jie Zhang <jie.zhang@analog.com>
> Signed-off-by: Robin Getz <robin.getz@analog.com>
> Signed-off-by: Mike Frysinger <vapier@gentoo.org>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Acked-by: Paul Mundt <lethal@linux-sh.org>

Acked-by: Greg Ungerer <gerg@uclinux.org>


> ---
> v3
> - tweak kconfig desc
>
> Documentation/nommu-mmap.txt | 26 ++++++++++++++++++++++++++
> fs/binfmt_elf_fdpic.c | 3 ++-
> include/asm-generic/mman-common.h | 5 +++++
> init/Kconfig | 22 ++++++++++++++++++++++
> mm/nommu.c | 8 +++++---
> 5 files changed, 60 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt
> index b565e82..8e1ddec 100644
> --- a/Documentation/nommu-mmap.txt
> +++ b/Documentation/nommu-mmap.txt
> @@ -119,6 +119,32 @@ FURTHER NOTES ON NO-MMU MMAP
> granule but will only discard the excess if appropriately configured as
> this has an effect on fragmentation.
>
> + (*) The memory allocated by a request for an anonymous mapping will normally
> + be cleared by the kernel before being returned in accordance with the
> + Linux man pages (ver 2.22 or later).
> +
> + In the MMU case this can be achieved with reasonable performance as
> + regions are backed by virtual pages, with the contents only being mapped
> + to cleared physical pages when a write happens on that specific page
> + (prior to which, the pages are effectively mapped to the global zero page
> + from which reads can take place). This spreads out the time it takes to
> + initialize the contents of a page - depending on the write-usage of the
> + mapping.
> +
> + In the no-MMU case, however, anonymous mappings are backed by physical
> + pages, and the entire map is cleared at allocation time. This can cause
> + significant delays during a userspace malloc() as the C library does an
> + anonymous mapping and the kernel then does a memset for the entire map.
> +
> + However, for memory that isn't required to be precleared - such as that
> + returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to
> + indicate to the kernel that it shouldn't bother clearing the memory before
> + returning it. Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled
> + to permit this, otherwise the flag will be ignored.
> +
> + uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
> + to allocate the brk and stack region.
> +
> (*) A list of all the private copy and anonymous mappings on the system is
> visible through /proc/maps in no-MMU mode.
>
> diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
> index 38502c6..79d2b1a 100644
> --- a/fs/binfmt_elf_fdpic.c
> +++ b/fs/binfmt_elf_fdpic.c
> @@ -380,7 +380,8 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm,
> down_write(&current->mm->mmap_sem);
> current->mm->start_brk = do_mmap(NULL, 0, stack_size,
> PROT_READ | PROT_WRITE | PROT_EXEC,
> - MAP_PRIVATE | MAP_ANONYMOUS | MAP_GROWSDOWN,
> + MAP_PRIVATE | MAP_ANONYMOUS |
> + MAP_UNINITIALIZED | MAP_GROWSDOWN,
> 0);
>
> if (IS_ERR_VALUE(current->mm->start_brk)) {
> diff --git a/include/asm-generic/mman-common.h b/include/asm-generic/mman-common.h
> index 5ee13b2..2011126 100644
> --- a/include/asm-generic/mman-common.h
> +++ b/include/asm-generic/mman-common.h
> @@ -19,6 +19,11 @@
> #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 MS_ASYNC 1 /* sync memory asynchronously */
> #define MS_INVALIDATE 2 /* invalidate the caches */
> diff --git a/init/Kconfig b/init/Kconfig
> index 09c5c64..309cd9a 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1069,6 +1069,28 @@ config SLOB
>
> endchoice
>
> +config MMAP_ALLOW_UNINITIALIZED
> + bool "Allow mmapped anonymous memory to be uninitialized"
> + depends on EMBEDDED && !MMU
> + default n
> + help
> + Normally, and according to the Linux spec, anonymous memory obtained
> + from mmap() has it's contents cleared before it is passed to
> + userspace. Enabling this config option allows you to request that
> + mmap() skip that if it is given an MAP_UNINITIALIZED flag, thus
> + providing a huge performance boost. If this option is not enabled,
> + then the flag will be ignored.
> +
> + This is taken advantage of by uClibc's malloc(), and also by
> + ELF-FDPIC binfmt's brk and stack allocator.
> +
> + Because of the obvious security issues, this option should only be
> + enabled on embedded devices where you control what is run in
> + userspace. Since that isn't generally a problem on no-MMU systems,
> + it is normally safe to say Y here.
> +
> + See Documentation/nommu-mmap.txt for more information.
> +
> config PROFILING
> bool "Profiling support (EXPERIMENTAL)"
> help
> diff --git a/mm/nommu.c b/mm/nommu.c
> index 5189b5a..11e8231 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -1143,9 +1143,6 @@ static int do_mmap_private(struct vm_area_struct *vma,
> if (ret < rlen)
> memset(base + ret, 0, rlen - ret);
>
> - } else {
> - /* if it's an anonymous mapping, then just clear it */
> - memset(base, 0, rlen);
> }
>
> return 0;
> @@ -1343,6 +1340,11 @@ unsigned long do_mmap_pgoff(struct file *file,
> goto error_just_free;
> add_nommu_region(region);
>
> + /* clear anonymous mappings that don't ask for uninitialized data */
> + if (!vma->vm_file && !(flags & MAP_UNINITIALIZED))
> + memset((void *)region->vm_start, 0,
> + region->vm_end - region->vm_start);
> +
> /* okay... we have a mapping; now we have to register it */
> result = vma->vm_start;
>

--
------------------------------------------------------------------------
Greg Ungerer -- Principal Engineer EMAIL: gerg@snapgear.com
SnapGear Group, McAfee PHONE: +61 7 3435 2888
825 Stanley St, FAX: +61 7 3891 3630
Woolloongabba, QLD, 4102, Australia WEB: http://www.SnapGear.com


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