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Date: Fri, 12 Apr 2024 04:14:16 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Peter Xu <peterx@redhat.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Lokesh Gidra <lokeshgidra@google.com>,
	Alistair Popple <apopple@nvidia.com>
Subject: Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks
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On Thu, Apr 11, 2024 at 11:02:32PM +0100, Matthew Wilcox wrote:
> > How many instructions it takes for a late RETRY for WRITEs to private file
> > mappings, fallback to mmap_sem?
> 
> Doesn't matter.  That happens _once_ per VMA, and it's dwarfed by the
> cost of allocating and initialising the COWed page.  You're adding
> instructions to every single page fault.  I'm not happy that we had to
> add extra instructions to the fault path for single-threaded programs,
> but we at least had the justification that we were improving scalability
> on large systems.  Your excuse is "it makes the code cleaner".  And
> honestly, I don't think it even does that.

Suren, what would you think to this?

diff --git a/mm/memory.c b/mm/memory.c
index 6e2fe960473d..e495adcbe968 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5821,15 +5821,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
        if (!vma_start_read(vma))
                goto inval;

-       /*
-        * find_mergeable_anon_vma uses adjacent vmas which are not locked.
-        * This check must happen after vma_start_read(); otherwise, a
-        * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA
-        * from its anon_vma.
-        */
-       if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
-               goto inval_end_read;
-
        /* Check since vm_start/vm_end might change before we lock the VMA */
        if (unlikely(address < vma->vm_start || address >= vma->vm_end))
                goto inval_end_read;

That takes a few insns out of the page fault path (good!) at the cost
of one extra trip around the fault handler for the first fault on an
anon vma.  It makes the file & anon paths more similar to each other
(good!)

We'd need some data to be sure it's really a win, but less code is
always good.

We could even eagerly initialise vma->anon_vma for anon vmas.  I don't
know why we don't do that.