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

[Andrew Morton <akpm@osdl.org>]

Andrea Arcangeli <andrea@suse.de> wrote:
>
> 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.

<looks>

Actually I was a bit harsh to the !CONFIG_PREEMPT case in unmap_vmas():

/* No preempt: go for the best straight-line efficiency */
#if !defined(CONFIG_PREEMPT)
#define ZAP_BLOCK_SIZE	(~(0UL))
#endif

We should probably make that 1024*PAGE_SIZE or something like that.

truncate_inode_pages() does a cond_resched() every 16 pages irrespective of
CONFIG_PREEMPT.