Re: [PATCH 0/9] Avoid populating unbounded num of ptes with mmap_sem held

From: Michel Lespinasse
Date: Fri Dec 21 2012 - 05:46:23 EST

Hi Greg,

I wanted to ask if you could check the sanity of the following patches
in nommu configurations. My understanding is that these always
populate mappings when they are created, so that MAP_POPULATE and
MAP_LOCKED are actually no-ops. Is this an accurate description ?


On Thu, Dec 20, 2012 at 4:49 PM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
> We have many vma manipulation functions that are fast in the typical case,
> but can optionally be instructed to populate an unbounded number of ptes
> within the region they work on:
> - mmap with MAP_POPULATE or MAP_LOCKED flags;
> - remap_file_pages() with MAP_NONBLOCK not set or when working on a
> VM_LOCKED vma;
> - mmap_region() and all its wrappers when mlock(MCL_FUTURE) is in effect;
> - brk() when mlock(MCL_FUTURE) is in effect.
> Current code handles these pte operations locally, while the sourrounding
> code has to hold the mmap_sem write side since it's manipulating vmas.
> This means we're doing an unbounded amount of pte population work with
> mmap_sem held, and this causes problems as Andy Lutomirski reported
> (we've hit this at Google as well, though it's not entirely clear why
> people keep trying to use mlock(MCL_FUTURE) in the first place).
> I propose introducing a new mm_populate() function to do this pte
> population work after the mmap_sem has been released. mm_populate()
> does need to acquire the mmap_sem read side, but critically, it
> doesn't need to hold continuously for the entire duration of the
> operation - it can drop it whenever things take too long (such as when
> hitting disk for a file read) and re-acquire it later on.
> The following patches are against v3.7:
> - Patches 1-2 fix some issues I noticed while working on the existing code.
> If needed, they could potentially go in before the rest of the patches.
> - Patch 3 introduces the new mm_populate() function and changes
> mmap_region() call sites to use it after they drop mmap_sem. This is
> inspired from Andy Lutomirski's proposal and is built as an extension
> of the work I had previously done for mlock() and mlockall() around
> v2.6.38-rc1. I had tried doing something similar at the time but had
> given up as there were so many do_mmap() call sites; the recent cleanups
> by Linus and Viro are a tremendous help here.
> - Patches 4-6 convert some of the less-obvious places doing unbounded
> pte populates to the new mm_populate() mechanism.
> - Patches 7-8 are code cleanups that are made possible by the
> mm_populate() work. In particular, they remove more code than the
> entire patch series added, which should be a good thing :)
> - Patch 9 is optional to this entire series. It only helps to deal more
> nicely with racy userspace programs that might modify their mappings
> while we're trying to populate them. It adds a new VM_POPULATE flag
> on the mappings we do want to populate, so that if userspace replaces
> them with mappings it doesn't want populated, mm_populate() won't
> populate those replacement mappings.
> Michel Lespinasse (9):
> mm: make mlockall preserve flags other than VM_LOCKED in def_flags
> mm: remap_file_pages() fixes
> mm: introduce mm_populate() for populating new vmas
> mm: use mm_populate() for blocking remap_file_pages()
> mm: use mm_populate() when adjusting brk with MCL_FUTURE in effect.
> mm: use mm_populate() for mremap() of VM_LOCKED vmas
> mm: remove flags argument to mmap_region
> mm: directly use __mlock_vma_pages_range() in find_extend_vma()
> mm: introduce VM_POPULATE flag to better deal with racy userspace programs
> arch/tile/mm/elf.c | 1 -
> fs/aio.c | 6 +++-
> include/linux/mm.h | 23 +++++++++---
> include/linux/mman.h | 4 ++-
> ipc/shm.c | 12 ++++---
> mm/fremap.c | 51 ++++++++++++++-------------
> mm/internal.h | 4 +-
> mm/memory.c | 24 -------------
> mm/mlock.c | 94 +++++++++++++------------------------------------
> mm/mmap.c | 77 ++++++++++++++++++++++++----------------
> mm/mremap.c | 25 +++++++------
> mm/nommu.c | 5 ++-
> mm/util.c | 6 +++-
> 13 files changed, 154 insertions(+), 178 deletions(-)
> --

Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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