linux-kernel - Re: [PATCH] mm: Always sanity check anon

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Date: Wed, 10 Apr 2024 22:10:45 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Lokesh Gidra <lokeshgidra@...gle.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks

On Wed, Apr 10, 2024 at 04:43:51PM -0400, Peter Xu wrote:
> On Wed, Apr 10, 2024 at 09:26:45PM +0100, Matthew Wilcox wrote:
> > On Wed, Apr 10, 2024 at 01:06:21PM -0400, Peter Xu wrote:
> > > anon_vma is a tricky object in the context of per-vma lock, because it's
> > > racy to modify it in that context and mmap lock is needed if it's not
> > > stable yet.
> > 
> > I object to this commit message.  First, it's not a "sanity check".  It's
> > a check to see if we already have an anon VMA.  Second, it's not "racy
> > to modify it" at all.  The problem is that we need to look at other
> > VMAs, for which we do not hold the lock.
> 
> For that "do not hold locks" part, isn't that "racy"?

No.

> > >   - We may always use mmap lock for the initial READs on a private file
> > >   mappings, while before this patch it _can_ (only when no WRITE ever
> > >   happened... but it doesn't make much sense for a MAP_PRIVATE..) do the
> > >   read fault with per-vma lock.
> > 
> > But that's a super common path!  Look at 'cat /proc/self/maps'.  All
> > your program text (including libraries) is mapped PRIVATE, and never
> > written to (except by ptrace, I guess).
> > 
> > NAK this patch.
> 
> We're talking about any vma that will first benefit from a per-vma lock
> here, right?
> 
> I think it should be only relevant to some major VMA or bunch of VMAs that
> an userspace maps explicitly, then iiuc the goal is we want to reduce the
> cache bouncing of the lock when it used to be per-mm, by replacing it with
> a finer lock.  It doesn't sound right that these libraries even fall into
> this category as they should just get loaded soon enough when the program
> starts.
> 
> IOW, my understanding is that per-vma lock doesn't benefit from such normal
> vmas or simple programs that much; we take either per-vma read lock, or
> mmap read lock, and I would expect similar performance when such cache
> bouncing isn't heavy.
> 
> I can do some tests later today or tomorrow. Any suggestion you have on
> amplifying such effect that you have concern with?

8 socket NUMA system, 800MB text segment, 10,000 threads.  No, I'm not
joking, that's a real customer workload.