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