Re: [RFC][PATCH 4/6] arm, mm: Convert arm to generic tlb

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, May 17, 2012 at 06:22:15PM +0100, Russell King wrote:
> On Thu, May 17, 2012 at 05:00:12PM +0100, Catalin Marinas wrote:
> > On Thu, May 17, 2012 at 12:28:06PM +0100, Peter Zijlstra wrote:
> > > On Thu, 2012-05-17 at 10:51 +0100, Russell King wrote:
> > > > On Thu, May 17, 2012 at 10:30:23AM +0100, Catalin Marinas wrote:
> > > > > Another minor thing is that on newer ARM processors (Cortex-A15) we
> > > > > need the TLB shootdown even on UP systems, so tlb_fast_mode should
> > > > > always return 0. Something like below (untested):
> > > > 
> > > > No Catalin, we need this for virtually all ARMv7 CPUs whether they're UP
> > > > or SMP, not just for A15, because of the speculative prefetch which can
> > > > re-load TLB entries from the page tables at _any_ time.
> > > 
> > > Hmm,. so this is mostly because of the confusion/coupling between
> > > tlb_remove_page() and tlb_remove_table() I guess. Since I don't see the
> > > freeing of the actual pages being a problem with speculative TLB
> > > reloads, just the page-tables.
> > > 
> > > Should we introduce a tlb_remove_table() regardless of
> > > HAVE_RCU_TABLE_FREE which always queues the tables regardless of
> > > tlb_fast_mode()? 
> > 
> > BTW, looking at your tlb-unify branch, does tlb_remove_table() call
> > tlb_flush/tlb_flush_mmu before freeing the tables?  I can only see
> > tlb_remove_page() doing this. On ARM, even UP, we need the TLB flushing
> > after clearing the pmd and before freeing the pte page table (and
> > ideally doing it less often than at every pte_free_tlb() call).
> 
> Catalin,
> 
> The way TLB shootdown stuff works is that _every_ single bit of memory
> which gets freed, whether its a page or a page table, gets added to a
> list of pages to be freed.
> 
> So, the sequence is:
> - remove pte/pmd/pud/pgd pointers
> - add pages, whether they be pages pointed to by pte entries or page tables
>   to be freed to a list
> - when list is sufficiently full, invalidate TLBs
> - free list of pages
> 
> That means the pages will not be freed, whether it be a page mapped
> into userspace or a page table until such time that the TLB has been
> invalidated.
> 
> For page tables, this is done via pXX_free_tlb(), which then calls out
> to the arch specific __pXX_free_tlb(), which ultimately then hands the
> page table over to tlb_remove_page() to add to the list of to-be-freed
> pages.

I know that already, not sure why you explained it again (but it's good
for future reference).

My point was that if we move to HAVE_RCU_FREE_TLB, the other
architectures doing this are calling tlb_remove_table() instead of
tlb_remove_page() in __p??_free_tlb(). And tlb_remove_table() does not
do any TLB maintenance when it can no longer queue pages (batch table
overflow).

-- 
Catalin

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