Re: [RFC 00/20] TLB batching consolidation and enhancements

[Nadav Amit <namit@vmware.com>]

> On Jan 30, 2021, at 7:30 PM, Nicholas Piggin <npiggin@gmail.com> wrote:
> 
> Excerpts from Nadav Amit's message of January 31, 2021 10:11 am:
>> From: Nadav Amit <namit@vmware.com>
>> 
>> There are currently (at least?) 5 different TLB batching schemes in the
>> kernel:
>> 
>> 1. Using mmu_gather (e.g., zap_page_range()).
>> 
>> 2. Using {inc|dec}_tlb_flush_pending() to inform other threads on the
>>   ongoing deferred TLB flush and flushing the entire range eventually
>>   (e.g., change_protection_range()).
>> 
>> 3. arch_{enter|leave}_lazy_mmu_mode() for sparc and powerpc (and Xen?).
>> 
>> 4. Batching per-table flushes (move_ptes()).
>> 
>> 5. By setting a flag on that a deferred TLB flush operation takes place,
>>   flushing when (try_to_unmap_one() on x86).
>> 
>> It seems that (1)-(4) can be consolidated. In addition, it seems that
>> (5) is racy. It also seems there can be many redundant TLB flushes, and
>> potentially TLB-shootdown storms, for instance during batched
>> reclamation (using try_to_unmap_one()) if at the same time mmu_gather
>> defers TLB flushes.
>> 
>> More aggressive TLB batching may be possible, but this patch-set does
>> not add such batching. The proposed changes would enable such batching
>> in a later time.
>> 
>> Admittedly, I do not understand how things are not broken today, which
>> frightens me to make further batching before getting things in order.
>> For instance, why is ok for zap_pte_range() to batch dirty-PTE flushes
>> for each page-table (but not in greater granularity). Can't
>> ClearPageDirty() be called before the flush, causing writes after
>> ClearPageDirty() and before the flush to be lost?
> 
> Because it's holding the page table lock which stops page_mkclean from 
> cleaning the page. Or am I misunderstanding the question?

Thanks. I understood this part. Looking again at the code, I now understand
my confusion: I forgot that the reverse mapping is removed after the PTE is
zapped.

Makes me wonder whether it is ok to defer the TLB flush to tlb_finish_mmu(),
by performing set_page_dirty() for the batched pages when needed in
tlb_finish_mmu() [ i.e., by marking for each batched page whether
set_page_dirty() should be issued for that page while collecting them ].

> I'll go through the patches a bit more closely when they all come 
> through. Sparc and powerpc of course need the arch lazy mode to get 
> per-page/pte information for operations that are not freeing pages, 
> which is what mmu gather is designed for.

IIUC you mean any PTE change requires a TLB flush. Even setting up a new PTE
where no previous PTE was set, right?

> I wouldn't mind using a similar API so it's less of a black box when 
> reading generic code, but it might not quite fit the mmu gather API
> exactly (most of these paths don't want a full mmu_gather on stack).

I see your point. It may be possible to create two mmu_gather structs: a
small one that only holds the flush information and another that also holds
the pages. 

>> This patch-set therefore performs the following changes:
>> 
>> 1. Change mprotect, task_mmu and mapping_dirty_helpers to use mmu_gather
>>   instead of {inc|dec}_tlb_flush_pending().
>> 
>> 2. Avoid TLB flushes if PTE permission is not demoted.
>> 
>> 3. Cleans up mmu_gather to be less arch-dependant.
>> 
>> 4. Uses mm's generations to track in finer granularity, either per-VMA
>>   or per page-table, whether a pending mmu_gather operation is
>>   outstanding. This should allow to avoid some TLB flushes when KSM or
>>   memory reclamation takes place while another operation such as
>>   munmap() or mprotect() is running.
>> 
>> 5. Changes try_to_unmap_one() flushing scheme, as the current seems
>>   broken to track in a bitmap which CPUs have outstanding TLB flushes
>>   instead of having a flag.
> 
> Putting fixes first, and cleanups and independent patches (like #2) next
> would help with getting stuff merged and backported.

I tried to do it mostly this way. There are some theoretical races which
I did not manage (or try hard enough) to create, so I did not include in
the “fixes” section. I will restructure the patch-set according to the
feedback.

Thanks,
Nadav