mm/hugetlb: fix excessive IPI broadcasts when unsharing PMD tables using mmu_gather

As reported, ever since commit 1013af4f585f ("mm/hugetlb: fix
huge_pmd_unshare() vs GUP-fast race") we can end up in some situations
where we perform so many IPI broadcasts when unsharing hugetlb PMD page
tables that it severely regresses some workloads.

In particular, when we fork()+exit(), or when we munmap() a large
area backed by many shared PMD tables, we perform one IPI broadcast per
unshared PMD table.

There are two optimizations to be had:

(1) When we process (unshare) multiple such PMD tables, such as during
    exit(), it is sufficient to send a single IPI broadcast (as long as
    we respect locking rules) instead of one per PMD table.

    Locking prevents that any of these PMD tables could get reused before
    we drop the lock.

(2) When we are not the last sharer (> 2 users including us), there is
    no need to send the IPI broadcast. The shared PMD tables cannot
    become exclusive (fully unshared) before an IPI will be broadcasted
    by the last sharer.

    Concurrent GUP-fast could walk into a PMD table just before we
    unshared it. It could then succeed in grabbing a page from the
    shared page table even after munmap() etc succeeded (and supressed
    an IPI). But there is not difference compared to GUP-fast just
    sleeping for a while after grabbing the page and re-enabling IRQs.

    Most importantly, GUP-fast will never walk into page tables that are
    no-longer shared, because the last sharer will issue an IPI
    broadcast.

    (if ever required, checking whether the PUD changed in GUP-fast
     after grabbing the page like we do in the PTE case could handle
     this)

So let's rework PMD sharing TLB flushing + IPI sync to use the mmu_gather
infrastructure so we can implement these optimizations and demystify the
code at least a bit. Extend the mmu_gather infrastructure to be able to
deal with our special hugetlb PMD table sharing implementation.

To make initialization of the mmu_gather easier when working on a single
VMA (in particular, when dealing with hugetlb), provide
tlb_gather_mmu_vma().

We'll consolidate the handling for (full) unsharing of PMD tables in
tlb_unshare_pmd_ptdesc() and tlb_flush_unshared_tables(), and track
in "struct mmu_gather" whether we had (full) unsharing of PMD tables.

Because locking is very special (concurrent unsharing+reuse must be
prevented), we disallow deferring flushing to tlb_finish_mmu() and instead
require an explicit earlier call to tlb_flush_unshared_tables().

From hugetlb code, we call huge_pmd_unshare_flush() where we make sure
that the expected lock protecting us from concurrent unsharing+reuse is
still held.

Check with a VM_WARN_ON_ONCE() in tlb_finish_mmu() that
tlb_flush_unshared_tables() was properly called earlier.

Document it all properly.

Notes about tlb_remove_table_sync_one() interaction with unsharing:

There are two fairly tricky things:

(1) tlb_remove_table_sync_one() is a NOP on architectures without
    CONFIG_MMU_GATHER_RCU_TABLE_FREE.

    Here, the assumption is that the previous TLB flush would send an
    IPI to all relevant CPUs. Careful: some architectures like x86 only
    send IPIs to all relevant CPUs when tlb->freed_tables is set.

    The relevant architectures should be selecting
    MMU_GATHER_RCU_TABLE_FREE, but x86 might not do that in stable
    kernels and it might have been problematic before this patch.

    Also, the arch flushing behavior (independent of IPIs) is different
    when tlb->freed_tables is set. Do we have to enlighten them to also
    take care of tlb->unshared_tables? So far we didn't care, so
    hopefully we are fine. Of course, we could be setting
    tlb->freed_tables as well, but that might then unnecessarily flush
    too much, because the semantics of tlb->freed_tables are a bit
    fuzzy.

    This patch changes nothing in this regard.

(2) tlb_remove_table_sync_one() is not a NOP on architectures with
    CONFIG_MMU_GATHER_RCU_TABLE_FREE that actually don't need a sync.

    Take x86 as an example: in the common case (!pv, !X86_FEATURE_INVLPGB)
    we still issue IPIs during TLB flushes and don't actually need the
    second tlb_remove_table_sync_one().

    This optimized can be implemented on top of this, by checking e.g., in
    tlb_remove_table_sync_one() whether we really need IPIs. But as
    described in (1), it really must honor tlb->freed_tables then to
    send IPIs to all relevant CPUs.

Notes on TLB flushing changes:

(1) Flushing for non-shared PMD tables

    We're converting from flush_hugetlb_tlb_range() to
    tlb_remove_huge_tlb_entry(). Given that we properly initialize the
    MMU gather in tlb_gather_mmu_vma() to be hugetlb aware, similar to
    __unmap_hugepage_range(), that should be fine.

(2) Flushing for shared PMD tables

    We're converting from various things (flush_hugetlb_tlb_range(),
    tlb_flush_pmd_range(), flush_tlb_range()) to tlb_flush_pmd_range().

    tlb_flush_pmd_range() achieves the same that
    tlb_remove_huge_tlb_entry() would achieve in these scenarios.
    Note that tlb_remove_huge_tlb_entry() also calls
    __tlb_remove_tlb_entry(), however that is only implemented on
    powerpc, which does not support PMD table sharing.

    Similar to (1), tlb_gather_mmu_vma() should make sure that TLB
    flushing keeps on working as expected.

Further, note that the ptdesc_pmd_pts_dec() in huge_pmd_share() is not a
concern, as we are holding the i_mmap_lock the whole time, preventing
concurrent unsharing. That ptdesc_pmd_pts_dec() usage will be removed
separately as a cleanup later.

There are plenty more cleanups to be had, but they have to wait until
this is fixed.

[david@kernel.org: fix kerneldoc]
  Link: https://lkml.kernel.org/r/f223dd74-331c-412d-93fc-69e360a5006c@kernel.org
Link: https://lkml.kernel.org/r/20251223214037.580860-5-david@kernel.org
Fixes: 1013af4f585f ("mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race")
Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reported-by: Uschakow, Stanislav" <suschako@amazon.de>
Closes: https://lore.kernel.org/all/4d3878531c76479d9f8ca9789dc6485d@amazon.de/
Tested-by: Laurence Oberman <loberman@redhat.com>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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