From: Hugh Dickins <hughd@google.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Hugh Dickins <hughd@google.com>, Peter Xu <peterx@redhat.com>,
Jann Horn <jannh@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp() versus uffd
Date: Tue, 22 Aug 2023 12:10:46 -0700 (PDT) [thread overview]
Message-ID: <3f926b9-ee3c-5cae-d7ad-9b3e1986bcbf@google.com> (raw)
In-Reply-To: <ZOUCVRxM8aPH6fva@casper.infradead.org>
On Tue, 22 Aug 2023, Matthew Wilcox wrote:
> 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.
Whilst I'm still happy with my 0-CPU wrist-watch, I do think you're right:
that SPLIT_PTLOCK_CPUS business was really just a safety-valve for when
introducing split ptlock in the first place, 4 pulled out of a hat, and
the unsplit ptlock path quite under-tested.
But I'll leave it to someone else do the job of removing it whenever.
Hugh
next prev parent reply other threads:[~2023-08-22 19:11 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-21 19:51 Hugh Dickins
2023-08-21 21:26 ` Peter Xu
2023-08-21 21:59 ` Jann Horn
2023-08-22 2:51 ` Hugh Dickins
2023-08-22 14:31 ` Peter Xu
2023-08-22 18:34 ` Hugh Dickins
2023-08-22 18:45 ` Matthew Wilcox
2023-08-22 19:10 ` Hugh Dickins [this message]
2023-08-22 14:39 ` Jann Horn
2023-08-22 15:22 ` Matthew Wilcox
2023-08-22 15:30 ` Jann Horn
2023-08-22 15:33 ` David Hildenbrand
2023-08-22 15:28 ` David Hildenbrand
2023-08-22 18:54 ` Hugh Dickins
2023-08-22 19:07 ` Jann Horn
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