Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI only to CPUs in kernel mode

[David Hildenbrand]

On 06.04.23 20:27, Peter Zijlstra wrote:
> On Thu, Apr 06, 2023 at 05:51:52PM +0200, David Hildenbrand wrote:
> > On 06.04.23 17:02, Peter Zijlstra wrote:
>
> > > DavidH, what do you thikn about reviving Jann's patches here:
> > >
> > >     https://bugs.chromium.org/p/project-zero/issues/detail?id=2365#c1
> > >
> > > Those are far more invasive, but afaict they seem to do the right thing.
> >
> > I recall seeing those while discussed on security@xxxxxxxxxx. What we
> > currently have was (IMHO for good reasons) deemed better to fix the issue,
> > especially when caring about backports and getting it right.
>
> Yes, and I think that was the right call. However, we can now revisit
> without having the pressure of a known defect and backport
> considerations.
>
> > The alternative that was discussed in that context IIRC was to simply
> > allocate a fresh page table, place the fresh page table into the list
> > instead, and simply free the old page table (then using common machinery).
> >
> > TBH, I'd wish (and recently raised) that we could just stop wasting memory
> > on page tables for THPs that are maybe never going to get PTE-mapped ... and
> > eventually just allocate on demand (with some caching?) and handle the
> > places where we're OOM and cannot PTE-map a THP in some descend way.
> >
> > ... instead of trying to figure out how to deal with these page tables we
> > cannot free but have to special-case simply because of GUP-fast.
>
> Not keeping them around sounds good to me, but I'm not *that* familiar
> with the THP code, most of that happened after I stopped tracking mm. So
> I'm not sure how feasible is it.
>
> But it does look entirely feasible to rework this page-table freeing
> along the lines Jann did.

It's most probably more feasible, although the easiest would be to just 
allocate a fresh page table to deposit and free the old one using the 
mmu gatherer.

This way we can avoid the khugepaged of tlb_remove_table_smp_sync(), but 
not the tlb_remove_table_one() usage. I suspect khugepaged isn't really 
relevant in RT kernels (IIRC, most of RT setups disable THP completely).

tlb_remove_table_one() only triggers if __get_free_page(GFP_NOWAIT | 
__GFP_NOWARN); fails. IIUC, that can happen easily under memory pressure 
because it doesn't wait for direct reclaim.

I don't know much about RT workloads (so I'd appreciate some feedback), 
but I guess we can run int memory pressure as well due to some !rt 
housekeeping task on the system?

--
Thanks,

David / dhildenb