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

[Greg Ungerer <gregungerer@westnet.com.au>]

Hi Michel,

On 12/21/2012 08:46 PM, Michel Lespinasse wrote:
> I wanted to ask if you could check the sanity of the following patches
> in nommu configurations. My understanding is that these always

Sure. I think it is worth CC'ing David Howells on these as well,
he has spent a fair bit of time in the mmap code for nommu.

Regards
Greg


> populate mappings when they are created, so that MAP_POPULATE and
> MAP_LOCKED are actually no-ops. Is this an accurate description ?
>
> Thanks,
>
> On Thu, Dec 20, 2012 at 4:49 PM, Michel Lespinasse <walken@google.com> 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(-)
>>
>> --
>> 1.7.7.3
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