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    SubjectRe: [RFC PATCH 1/8] mm: Switch mm->get_unmapped_area() to a flag
    On Thu, Feb 15, 2024 at 03:13:25PM -0800, Rick Edgecombe wrote:
    > The mm_struct contains a function pointer *get_unmapped_area(), which
    > is set to either arch_get_unmapped_area() or
    > arch_get_unmapped_area_topdown() during the initialization of the mm.
    >
    > Since the function pointer only ever points to two functions that are named
    > the same across all arch's, a function pointer is not really required. In
    > addition future changes will want to add versions of the functions that
    > take additional arguments. So to save a pointers worth of bytes in
    > mm_struct, and prevent adding additional function pointers to mm_struct in
    > future changes, remove it and keep the information about which
    > get_unmapped_area() to use in a flag.
    >
    > Introduce a helper, mm_get_unmapped_area(), to easily convert code that
    > refers to the old function pointer to instead select and call either
    > arch_get_unmapped_area() or arch_get_unmapped_area_topdown() based on the
    > flag. Then drop the mm->get_unmapped_area() function pointer. Leave the
    > get_unmapped_area() pointer in struct file_operations alone. The main
    > purpose of this change is to reorganize in preparation for future changes,
    > but it also converts the calls of mm->get_unmapped_area() from indirect
    > branches into a direct ones.
    >
    > The stress-ng bigheap benchmark calls realloc a lot, which calls through
    > get_unmapped_area() in the kernel. On x86, the change yielded a ~4%
    > improvement there. (bogo ops/s (usr+sys time))
    >
    > In testing a few x86 configs, removing the pointer unfortunately didn't
    > result in any actual size reductions in the compiled layout of mm_struct.
    > But depending on compiler or arch alignment requirements, the change could
    > possibly shrink the size of mm_struct.
    >
    > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    > ---
    > arch/s390/mm/hugetlbpage.c | 2 +-
    > arch/s390/mm/mmap.c | 4 ++--
    > arch/sparc/kernel/sys_sparc_64.c | 15 ++++++---------
    > arch/sparc/mm/hugetlbpage.c | 2 +-
    > arch/x86/kernel/cpu/sgx/driver.c | 2 +-
    > arch/x86/mm/hugetlbpage.c | 2 +-
    > arch/x86/mm/mmap.c | 4 ++--
    > drivers/char/mem.c | 2 +-
    > drivers/dax/device.c | 6 +++---
    > fs/hugetlbfs/inode.c | 2 +-
    > fs/proc/inode.c | 15 ++++++++-------
    > fs/ramfs/file-mmu.c | 2 +-
    > include/linux/mm_types.h | 6 +-----
    > include/linux/sched/coredump.h | 1 +
    > include/linux/sched/mm.h | 5 +++++
    > io_uring/io_uring.c | 2 +-
    > mm/debug.c | 6 ------
    > mm/huge_memory.c | 6 +++---
    > mm/mmap.c | 21 ++++++++++++++++++---
    > mm/shmem.c | 11 +++++------
    > mm/util.c | 6 +++---
    > 21 files changed, 65 insertions(+), 57 deletions(-)
    >
    > diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
    > index 02f5090ffea2..428e440424c5 100644
    > --- a/include/linux/sched/coredump.h
    > +++ b/include/linux/sched/coredump.h
    > @@ -74,6 +74,7 @@ static inline int get_dumpable(struct mm_struct *mm)
    > #define MMF_DISABLE_THP_MASK (1 << MMF_DISABLE_THP)
    > #define MMF_OOM_REAP_QUEUED 25 /* mm was queued for oom_reaper */
    > #define MMF_MULTIPROCESS 26 /* mm is shared between processes */
    > +#define MMF_TOPDOWN 27 /* mm is shared between processes */

    Nit: you may want to update the comment here ;-)

    > /*
    > * MMF_HAS_PINNED: Whether this mm has pinned any pages. This can be either
    > * replaced in the future by mm.pinned_vm when it becomes stable, or grow into

    --
    Sincerely yours,
    Mike.

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