Re: anon-vma (and now filebacked-mappings too) mprotect vma merging [Re: 2.6.5-rc2-aa vma merging]

[Andrea Arcangeli]

On Sat, Apr 03, 2004 at 05:13:58PM -0800, Andrew Morton wrote:
> That change was made for scheduling latency reasons, mainly due to huge
> pagetable walks in zap_page_range():
> 
>     i_shared_lock is held for a very long time during vmtruncate() and
>     causes high scheduling latencies when truncating a file which is
>     mmapped.  I've seen 100 milliseconds.
> 
>     So turn it into a semaphore.  It nests inside mmap_sem.  This
>     change is also needed by the shared pagetable patch, which needs to
>     unshare pte's on the vmtruncate path - lots of pagetable pages need to
>     be allocated and they are using __GFP_WAIT.
> 
> If we no longer hold that lock during pte takedown then sure, it would
> be better if we had a spinlock in there.

agreed, vmtruncate may still be very costly due the pte scan, so
I was probably wrong about not needing the semaphore anymore with
prio-tree (I was too objrmap centric, and after all objrmap with the
trylocks threats it like a spinlock anyways), though the semaphore
cannot help the latency of the non-contention case (I mean with preempt
disabled), that seem to have room for improvements with a cond_sched()
before returning from invalidate_mmap_range_list not present yet.
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