Re: [RESEND PATCH V2 1/3] Add mmap flag to request pages are locked after page fault

From: Michal Hocko
Date: Thu Jun 18 2015 - 11:29:16 EST

[Sorry for the late reply - I meant to answer in the previous threads
but something always preempted me from that]

On Wed 10-06-15 09:26:48, Eric B Munson wrote:
> The cost of faulting in all memory to be locked can be very high when
> working with large mappings. If only portions of the mapping will be
> used this can incur a high penalty for locking.
>
> For the example of a large file, this is the usage pattern for a large
> statical language model (probably applies to other statical or graphical
> models as well). For the security example, any application transacting
> in data that cannot be swapped out (credit card data, medical records,
> etc).

Such a use case makes some sense to me but I am not sure the way you
implement it is the right one. This is another mlock related flag for
mmap with a different semantic. You do not want to prefault but e.g. is
the readahead or fault around acceptable? I do not see anything in your
patch to handle those...

Wouldn't it be much more reasonable and straightforward to have
MAP_FAULTPOPULATE as a counterpart for MAP_POPULATE which would
explicitly disallow any form of pre-faulting? It would be usable for
other usecases than with MAP_LOCKED combination.

> This patch introduces the ability to request that pages are not
> pre-faulted, but are placed on the unevictable LRU when they are finally
> faulted in.
>
> To keep accounting checks out of the page fault path, users are billed
> for the entire mapping lock as if MAP_LOCKED was used.
>
> Signed-off-by: Eric B Munson <emunson@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: linux-alpha@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-mips@xxxxxxxxxxxxxx
> Cc: linux-parisc@xxxxxxxxxxxxxxx
> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> Cc: sparclinux@xxxxxxxxxxxxxxx
> Cc: linux-xtensa@xxxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-arch@xxxxxxxxxxxxxxx
> Cc: linux-api@xxxxxxxxxxxxxxx
> ---
> arch/alpha/include/uapi/asm/mman.h | 1 +
> arch/mips/include/uapi/asm/mman.h | 1 +
> arch/parisc/include/uapi/asm/mman.h | 1 +
> arch/powerpc/include/uapi/asm/mman.h | 1 +
> arch/sparc/include/uapi/asm/mman.h | 1 +
> arch/tile/include/uapi/asm/mman.h | 1 +
> arch/xtensa/include/uapi/asm/mman.h | 1 +
> include/linux/mm.h | 1 +
> include/linux/mman.h | 3 ++-
> include/uapi/asm-generic/mman.h | 1 +
> mm/mmap.c | 4 ++--
> mm/swap.c | 3 ++-
> 12 files changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> index 0086b47..15e96e1 100644
> --- a/arch/alpha/include/uapi/asm/mman.h
> +++ b/arch/alpha/include/uapi/asm/mman.h
> @@ -30,6 +30,7 @@
> #define MAP_NONBLOCK 0x40000 /* do not block on IO */
> #define MAP_STACK 0x80000 /* give out an address that is best suited for process/thread stacks */
> #define MAP_HUGETLB 0x100000 /* create a huge page mapping */
> +#define MAP_LOCKONFAULT 0x200000 /* Lock pages after they are faulted in, do not prefault */
>
> #define MS_ASYNC 1 /* sync memory asynchronously */
> #define MS_SYNC 2 /* synchronous memory sync */
> diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h
> index cfcb876..47846a5 100644
> --- a/arch/mips/include/uapi/asm/mman.h
> +++ b/arch/mips/include/uapi/asm/mman.h
> @@ -48,6 +48,7 @@
> #define MAP_NONBLOCK 0x20000 /* do not block on IO */
> #define MAP_STACK 0x40000 /* give out an address that is best suited for process/thread stacks */
> #define MAP_HUGETLB 0x80000 /* create a huge page mapping */
> +#define MAP_LOCKONFAULT 0x100000 /* Lock pages after they are faulted in, do not prefault */
>
> /*
> * Flags for msync
> diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h
> index 294d251..1514cd7 100644
> --- a/arch/parisc/include/uapi/asm/mman.h
> +++ b/arch/parisc/include/uapi/asm/mman.h
> @@ -24,6 +24,7 @@
> #define MAP_NONBLOCK 0x20000 /* do not block on IO */
> #define MAP_STACK 0x40000 /* give out an address that is best suited for process/thread stacks */
> #define MAP_HUGETLB 0x80000 /* create a huge page mapping */
> +#define MAP_LOCKONFAULT 0x100000 /* Lock pages after they are faulted in, do not prefault */
>
> #define MS_SYNC 1 /* synchronous memory sync */
> #define MS_ASYNC 2 /* sync memory asynchronously */
> diff --git a/arch/powerpc/include/uapi/asm/mman.h b/arch/powerpc/include/uapi/asm/mman.h
> index 6ea26df..fce74fe 100644
> --- a/arch/powerpc/include/uapi/asm/mman.h
> +++ b/arch/powerpc/include/uapi/asm/mman.h
> @@ -27,5 +27,6 @@
> #define MAP_NONBLOCK 0x10000 /* do not block on IO */
> #define MAP_STACK 0x20000 /* give out an address that is best suited for process/thread stacks */
> #define MAP_HUGETLB 0x40000 /* create a huge page mapping */
> +#define MAP_LOCKONFAULT 0x80000 /* Lock pages after they are faulted in, do not prefault */
>
> #endif /* _UAPI_ASM_POWERPC_MMAN_H */
> diff --git a/arch/sparc/include/uapi/asm/mman.h b/arch/sparc/include/uapi/asm/mman.h
> index 0b14df3..12425d8 100644
> --- a/arch/sparc/include/uapi/asm/mman.h
> +++ b/arch/sparc/include/uapi/asm/mman.h
> @@ -22,6 +22,7 @@
> #define MAP_NONBLOCK 0x10000 /* do not block on IO */
> #define MAP_STACK 0x20000 /* give out an address that is best suited for process/thread stacks */
> #define MAP_HUGETLB 0x40000 /* create a huge page mapping */
> +#define MAP_LOCKONFAULT 0x80000 /* Lock pages after they are faulted in, do not prefault */
>
>
> #endif /* _UAPI__SPARC_MMAN_H__ */
> diff --git a/arch/tile/include/uapi/asm/mman.h b/arch/tile/include/uapi/asm/mman.h
> index 81b8fc3..ec04eaf 100644
> --- a/arch/tile/include/uapi/asm/mman.h
> +++ b/arch/tile/include/uapi/asm/mman.h
> @@ -29,6 +29,7 @@
> #define MAP_DENYWRITE 0x0800 /* ETXTBSY */
> #define MAP_EXECUTABLE 0x1000 /* mark it as an executable */
> #define MAP_HUGETLB 0x4000 /* create a huge page mapping */
> +#define MAP_LOCKONFAULT 0x8000 /* Lock pages after they are faulted in, do not prefault */
>
>
> /*
> diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h
> index 201aec0..42d43cc 100644
> --- a/arch/xtensa/include/uapi/asm/mman.h
> +++ b/arch/xtensa/include/uapi/asm/mman.h
> @@ -55,6 +55,7 @@
> #define MAP_NONBLOCK 0x20000 /* do not block on IO */
> #define MAP_STACK 0x40000 /* give out an address that is best suited for process/thread stacks */
> #define MAP_HUGETLB 0x80000 /* create a huge page mapping */
> +#define MAP_LOCKONFAULT 0x100000 /* Lock pages after they are faulted in, do not prefault */
> #ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
> # define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be
> * uninitialized */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0755b9f..3e31457 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -126,6 +126,7 @@ extern unsigned int kobjsize(const void *objp);
> #define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */
> #define VM_DENYWRITE 0x00000800 /* ETXTBSY on write attempts.. */
>
> +#define VM_LOCKONFAULT 0x00001000 /* Lock the pages covered when they are faulted in */
> #define VM_LOCKED 0x00002000
> #define VM_IO 0x00004000 /* Memory mapped I/O or similar */
>
> diff --git a/include/linux/mman.h b/include/linux/mman.h
> index 16373c8..437264b 100644
> --- a/include/linux/mman.h
> +++ b/include/linux/mman.h
> @@ -86,7 +86,8 @@ 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_LOCKED, VM_LOCKED );
> + _calc_vm_trans(flags, MAP_LOCKED, VM_LOCKED ) |
> + _calc_vm_trans(flags, MAP_LOCKONFAULT,VM_LOCKONFAULT);
> }
>
> unsigned long vm_commit_limit(void);
> diff --git a/include/uapi/asm-generic/mman.h b/include/uapi/asm-generic/mman.h
> index e9fe6fd..fc4e586 100644
> --- a/include/uapi/asm-generic/mman.h
> +++ b/include/uapi/asm-generic/mman.h
> @@ -12,6 +12,7 @@
> #define MAP_NONBLOCK 0x10000 /* do not block on IO */
> #define MAP_STACK 0x20000 /* give out an address that is best suited for process/thread stacks */
> #define MAP_HUGETLB 0x40000 /* create a huge page mapping */
> +#define MAP_LOCKONFAULT 0x80000 /* Lock pages after they are faulted in, do not prefault */
>
> /* Bits [26:31] are reserved, see mman-common.h for MAP_HUGETLB usage */
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index bb50cac..ba1a6bf 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1233,7 +1233,7 @@ static inline int mlock_future_check(struct mm_struct *mm,
> unsigned long locked, lock_limit;
>
> /* mlock MCL_FUTURE? */
> - if (flags & VM_LOCKED) {
> + if (flags & (VM_LOCKED | VM_LOCKONFAULT)) {
> locked = len >> PAGE_SHIFT;
> locked += mm->locked_vm;
> lock_limit = rlimit(RLIMIT_MEMLOCK);
> @@ -1301,7 +1301,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
> vm_flags = calc_vm_prot_bits(prot) | calc_vm_flag_bits(flags) |
> mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
>
> - if (flags & MAP_LOCKED)
> + if (flags & (MAP_LOCKED | MAP_LOCKONFAULT))
> if (!can_do_mlock())
> return -EPERM;
>
> diff --git a/mm/swap.c b/mm/swap.c
> index a7251a8..07c905e 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -711,7 +711,8 @@ void lru_cache_add_active_or_unevictable(struct page *page,
> {
> VM_BUG_ON_PAGE(PageLRU(page), page);
>
> - if (likely((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) != VM_LOCKED)) {
> + if (likely((vma->vm_flags & (VM_LOCKED | VM_LOCKONFAULT)) == 0) ||
> + (vma->vm_flags & VM_SPECIAL)) {
> SetPageActive(page);
> lru_cache_add(page);
> return;
> --
> 1.9.1
>

--
Michal Hocko
SUSE Labs
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