Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SW trackingofdirty & young

From: Shan Hai
Date: Tue Jul 19 2011 - 04:42:59 EST

On 07/19/2011 04:26 PM, David Laight wrote:

Got it, if the fault_in_user_writeable() is designed to catch the
exact same write permission fault problem we discuss here, so
your patch fixed that very nicely, we should fixup it by directly
calling handle_mm_fault like what you did because we are for sure
to know what just happened(permission violation), its not necessary
to check what's happened by calling gup-->follow_page, and
further the follow_page failed to report the fault :-)
One thought I've had - and I don't know enough about the data
area in use to know if it is a problem - is what happens if
a different cpu faults on the same user page and has already
marked it 'valid' between the fault happening and the fault
handler looking at the page tables to find out why.
If any of the memory areas are shared, it might be that the
PTE (etc) might already show the page a writable by the
time the fault handler is looking at them - this might confuse it!

There is no problem at all if you mean *valid* by page present
and writable, because when the fault_in_user_writeable()
is called, the pte to the shared page was already setup by
demand paging and pte.present and pte.write was set, and the
reason why the fault was taken is that because of violation of
permission on present and writable user page occurred on sw
dirty/young tracking architectures.

Shan Hai


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