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

[David Hildenbrand <david@redhat.com>]

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@kernel.org. 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