linux-kernel - Re: [PATCH v2 1/1] mm: do not increment pgfault stats when page fault handler retries

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Message-ID: <ZD7ld4Iffdmv5Q8f@x1n>
Date:   Tue, 18 Apr 2023 14:46:15 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org,
        hannes@...xchg.org, mhocko@...e.com, josef@...icpanda.com,
        jack@...e.cz, ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        michel@...pinasse.org, liam.howlett@...cle.com, jglisse@...gle.com,
        vbabka@...e.cz, minchan@...gle.com, dave@...olabs.net,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 1/1] mm: do not increment pgfault stats when page
 fault handler retries

On Tue, Apr 18, 2023 at 09:45:52AM -0700, Suren Baghdasaryan wrote:
> On Tue, Apr 18, 2023 at 8:48 AM Peter Xu <peterx@...hat.com> wrote:
> >
> > On Tue, Apr 18, 2023 at 04:32:27PM +0100, Matthew Wilcox wrote:
> > > ... when we called clone()?  A thread by definition has a reference to
> > > its own mm.
> >
> > Ah yes.. thanks!
> 
> re: I also had a quick look on do_exit() but I also didn't see where
> do we e.g. wait for all the threads to stop before recycles a mm.
> 
> We recycle mm after all refcounts are dropped in the exit path:
>   do_exit
>     exit_mm
>       mmput(if !mm->mm_users)
>         mmdrop(if !mm->mm_count)
>           free_mm

I assume Matthew means when the task_struct is created with part of
kernel_clone().

copy_mm() has:

	if (clone_flags & CLONE_VM) {
		mmget(oldmm);
		mm = oldmm;
	} else {
		mm = dup_mm(tsk, current->mm);
		if (!mm)
			return -ENOMEM;
	}

If CLONE_VM, we'll mmget() on the existing mm. If !CLONE_VM, we'll just
create a new one with reference held.  For the latter, I think that hides
in mm_init() where it'll just set it to 1:

	atomic_set(&mm->mm_users, 1);

With mm_users>0, do_exit() will leave the mm_struct* alone since mmput()
will still be called but not the final step on mmdrop().

Thanks,

-- 
Peter Xu