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

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