Re: Fwd: Re: struct page field arrangement

["Jan Beulich" <jbeulich@novell.com>]

>>> Keir Fraser <keir@xensource.com> 01.03.07 11:22 >>>
>On 1/3/07 08:42, "Jan Beulich" <jbeulich@novell.com> wrote:
>
>> Having looked a little into the disabled SPLIT_PT_LOCK issue on Xen, I
>> realized that is shouldn't be too difficult to re-enable it (at least in some
>> cases).
>
>How does pagetable locking work in modern Linux kernels? It seems that
>updates of ptes are protected by a per-page lock or the mm lock, and
>population of page directory entries is protected by the mm lock, but that
>there is no synchronisation with read-only pagetable walks. Does this mean
>that sections of pagetable hierarchy are never reaped from a process until
>it dies?

Yes, that's my understanding.

>Can we confident that the mm_pin/mm_unpin code (which walks pagetables and
>has to find every page to make every one read-only or writable) is safe?
>Presumably for this to be true we need to be sure that noone can meanwhile
>concurrently be populating the pagetable we are walking with extra
>pgds/puds/pmds/ptes...

Since the pin/unpin walking only cares about pgd/pud/pmd entries, synchronization
is guaranteed through mm->page_table_lock. The pte lock is used only for leaf
entries, which are of no concern to (un)pinning.

Jan