Re: page table lock patch V15 [0/7]: overview

Christoph Lameter
2005-01-13T18:16:58Z

On Thu, 13 Jan 2005, Andi Kleen wrote:

> The rule in i386/x86-64 is that you cannot set the PTE in a non atomic way
> when its present bit is set (because the hardware could asynchronously
> change bits in the PTE that would get lost). Atomic way means clearing
> first and then replacing in an atomic operation.

Hmm. I replaced that portion in the swapper with an xchg operation
and inspect the result later. Clearing a pte and then setting it to
something would open a window for the page fault handler to set up a new
pte there since it does not take the page_table_lock. That xchg must be
atomic for PAE mode to work then.

> This helps you because you shouldn't be looking at the pte anyways
> when pte_present is false. When it is not false it is always updated
> atomically.

so pmd_present, pud_none and pgd_none could be considered atomic even if
the pm/u/gd_t is a multi-word entity? In that case the current approach
would work for higher level entities and in particular S/390 would be in
the clear.

But then the issues of replacing multi-word ptes on i386 PAE remain. If no
write lock is held on mmap_sem then all writes to pte's must be atomic in
order for the get_pte_atomic operation to work reliably.