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

Nicholas Piggin
2021-01-31T03:31:50Z

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?

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.

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).

>
> 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.

Thanks,
Nick