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

[Michel Lespinasse <walken@google.com>]

On Sun, Dec 16, 2012 at 10:05 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Sun, Dec 16, 2012 at 4:39 AM, Michel Lespinasse <walken@google.com> wrote:
>> My main concern is that just downgrading the mmap_sem only hides the
>> problem: as soon as a writer gets queued on that mmap_sem,
>> reader/writer fairness kicks in and blocks any new readers, which
>> makes the problem reappear. So in order to completely fix the issue,
>> we should look for a way that doesn't require holding the mmap_sem
>> (even in read mode) for the entire duration of the populate or mlock
>> operation.
>
> Ugh.
>
> At least with my patch, mmap in MCL_FUTURE mode is no worse than mmap
> + mlock.  I suspect I haven't hit this because all my mmaping is done
> by one thread, so it never ends up waiting for itself, and the other
> thread have very short mmap_sem hold times.

Yes, you won't hit the problems with long read-side mmap_sem hold
times if you don't have other threads blocking for the write side.

>> 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.

We can't easily release mmap_sem from within mmap_region() since
mmap_region's callers don't expect it; however we can defer the page
mlocking and we don't have to hold mmap_sem continuously until then.
The only constraints are the new VM_LOCKED region's pages must be
mlocked before we return to userspace, and that if a concurrent thread
modifies the mappings while we don't hold mmap_sem, and creates a new
non-mlocked region, we shouldn't mlock those pages in
do_mlock_pages().

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

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