linux-kernel - Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped

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Date:   Tue, 22 Aug 2023 19:45:41 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Peter Xu <peterx@...hat.com>, Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
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        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
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        Russell King <linux@...linux.org.uk>,
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        sparclinux@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp()
 versus uffd

On Tue, Aug 22, 2023 at 11:34:19AM -0700, Hugh Dickins wrote:
> (Yes, the locking is a bit confusing: but mainly for the unrelated reason,
> that with the split locking configs, we never quite know whether this lock
> is the same as that lock or not, and so have to be rather careful.)

Is it time to remove the PTE split locking config option?  I believe all
supported architectures have at least two levels of page tables, so if we
have split ptlocks, ptl and pml are always different from each other (it's
just that on two level machines, pmd == pud == p4d == pgd).  With huge
thread counts now being the norm, it's hard to see why anybody would want
to support SMP and !SPLIT_PTE_PTLOCKS.  To quote the documentation ...

  Split page table lock for PTE tables is enabled compile-time if
  CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS.
  If split lock is disabled, all tables are guarded by mm->page_table_lock.

You can barely buy a wrist-watch without eight CPUs these days.