linux-kernel - Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma

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Message-ID: <CAJuCfpHJn9jLT4zW2vPc4kv-Y3_3BTNXkn7pjFEKLVeFjxL4oQ@mail.gmail.com>
Date: Fri, 13 Dec 2024 09:45:33 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org, liam.howlett@...cle.com, 
	lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz, 
	hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com, 
	mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com, 
	oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org, brauner@...nel.org, 
	dhowells@...hat.com, hdanton@...a.com, hughd@...gle.com, minchan@...gle.com, 
	jannh@...gle.com, shakeel.butt@...ux.dev, souravpanda@...gle.com, 
	pasha.tatashin@...een.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	kernel-team@...roid.com
Subject: Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma_lock

On Fri, Dec 13, 2024 at 1:57 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Dec 12, 2024 at 08:48:52PM -0800, Suren Baghdasaryan wrote:
>
> > I'm not sure if this is the best way to deal with this circular
> > dependency. Any other ideas?
>
> Move the waiting into an out-of-line slow-path?
>
>   if (atomic_read(&vma->refcnt) != 2)
>     __vma_write_start_wait(mm, vma);

The problem is not a function but the addition of struct rcuwait into
struct mm_struct. The type should be known and I don't want to add it
via a pointer to avoid extra dereferences. After thinking it over, I
think the simplest way would be to move the definition of struct
rcuwait into a separate header and include that header in mm_types.h.
That, with some uninlining like you suggested, should do the trick.

>
> or somesuch..