linux-kernel - Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area

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Date:   Mon, 23 Jan 2023 10:56:32 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, vbabka@...e.cz, hannes@...xchg.org,
        mgorman@...hsingularity.net, dave@...olabs.net,
        peterz@...radead.org, ldufour@...ux.ibm.com,
        laurent.dufour@...ibm.com, paulmck@...nel.org, luto@...nel.org,
        songliubraving@...com, peterx@...hat.com, david@...hat.com,
        dhowells@...hat.com, hughd@...gle.com, bigeasy@...utronix.de,
        kent.overstreet@...ux.dev, punit.agrawal@...edance.com,
        lstoakes@...il.com, peterjung1337@...il.com, rientjes@...gle.com,
        axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com,
        jannh@...gle.com, shakeelb@...gle.com, tatashin@...gle.com,
        edumazet@...gle.com, gthelen@...gle.com, gurua@...gle.com,
        arjunroy@...gle.com, soheil@...gle.com, hughlynch@...gle.com,
        leewalsh@...gle.com, posk@...gle.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in
 vm_area_free

On Fri 20-01-23 09:50:01, Suren Baghdasaryan wrote:
> On Fri, Jan 20, 2023 at 9:32 AM Matthew Wilcox <willy@...radead.org> wrote:
[...]
> > The page fault handler (or whatever other reader -- ptrace, proc, etc)
> > should have a refcount on the mm_struct, so we can't be in this path
> > trying to free VMAs.  Right?
> 
> Hmm. That sounds right. I checked process_mrelease() as well, which
> operated on mm with only mmgrab()+mmap_read_lock() but it only unmaps
> VMAs without freeing them, so we are still good. Michal, do you agree
> this is ok?

Don't we need RCU procetions for the vma life time assurance? Jann has
already shown how rwsem is not safe wrt to unlock and free without RCU.

-- 
Michal Hocko
SUSE Labs