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

[Andrew Morton]

Andrea Arcangeli <andrea@xxxxxxx> wrote:
>
> BTW, now that the lookup on the
>  prio-tree is immediate the i_shared_sem has no reason anymore to be a
>  semaphore either, it should go back to a spinlock like in 2.4, like the
>  anon_vma is also protected by a spinlock.

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.

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