Re: [PATCH v2] mm: Downgrade mmap_sem before locking or populating on mmap

[Andy Lutomirski]

On Sun, Dec 16, 2012 at 7:29 PM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
> On Sun, Dec 16, 2012 at 10:05 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Sun, Dec 16, 2012 at 4:39 AM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
>>> I think this could be done by extending the mlock work I did as part
>>> of v2.6.38-rc1. The commit message for
>>> c explains the idea; basically
>>> mlock() was split into do_mlock() which just sets the VM_LOCKED flag
>>> on vmas as needed, and do_mlock_pages() which goes through a range of
>>> addresses and actually populates/mlocks each individual page that is
>>> part of a VM_LOCKED vma.
>>
>> Doesn't this have the same problem?  It holds mmap_sem for read for a
>> long time, and if another writer comes in then r/w starvation
>> prevention will kick in.
>
> Well, my point is that do_mlock_pages() doesn't need to hold the
> mmap_sem read side for a long time. It currently releases it when
> faulting a page requires a disk read, and could conceptually release
> it more often if needed.

I can't find this code.  It looks like do_mlock_pages calls
__mlock_vma_pages_range, which calls __get_user_pages, which makes its
way to __do_fault, which doesn't seem to drop mmap_sem.

--Andy
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