linux-kernel - Re: [PATCH v3 1/2] mm: protect free_pgtables with mmap_lock write lock in exit

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Date:   Wed, 8 Dec 2021 11:13:42 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michal Hocko <mhocko@...e.com>, akpm@...ux-foundation.org,
        rientjes@...gle.com, hannes@...xchg.org, guro@...com,
        riel@...riel.com, minchan@...nel.org, kirill@...temov.name,
        aarcange@...hat.com, christian@...uner.io, hch@...radead.org,
        oleg@...hat.com, david@...hat.com, jannh@...gle.com,
        shakeelb@...gle.com, luto@...nel.org, christian.brauner@...ntu.com,
        fweimer@...hat.com, jengelh@...i.de, timmurray@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 1/2] mm: protect free_pgtables with mmap_lock write
 lock in exit_mmap

On Wed, Dec 8, 2021 at 8:50 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Wed, Dec 8, 2021 at 8:05 AM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Wed, Dec 08, 2021 at 04:51:58PM +0100, Michal Hocko wrote:
> > > On Wed 08-12-21 15:01:24, Matthew Wilcox wrote:
> > > > On Tue, Dec 07, 2021 at 03:08:19PM -0800, Suren Baghdasaryan wrote:
> > > > > > >         /**
> > > > > > >          * @close: Called when the VMA is being removed from the MM.
> > > > > > >          * Context: Caller holds mmap_lock.
> > > > >
> > > > > BTW, is the caller always required to hold mmap_lock for write or it
> > > > > *might* hold it?
> > > >
> > > > __do_munmap() might hold it for read, thanks to:
> > > >
> > > >         if (downgrade)
> > > >                 mmap_write_downgrade(mm);
> > > >
> > > > Should probably say:
> > > >
> > > >     * Context: User context.  May sleep.  Caller holds mmap_lock.
> > > >
> > > > I don't think we should burden the implementor of the vm_ops with the
> > > > knowledge that the VM chooses to not hold the mmap_lock under certain
> > > > circumstances when it doesn't matter whether it's holding the mmap_lock
> > > > or not.
> > >
> > > If we document it like that some code might depend on that lock to be
> > > held. I think we only want to document that the holder itself is not
> > > allowed to take mmap sem or a depending lock.
> >
> > The only place where we're not currently holding the mmap_lock is at
> > task exit, where the mmap_lock is effectively held because nobody else
> > can modify the task's mm.  Besides, Suren is changing that in this patch
> > series anyway, so it will be always true.
>
> Ok, I'll make it a separate patch after the patch that changes
> exit_mmap and this statement will become always true. Sounds
> reasonable?

Actually, while today vma_ops->close is called with mmap_lock held, I
believe we want this comment to reflect the restrictions on the
callback itself, not on the user. IOW, we want to say that the
callback should not take mmap_lock while the caller might or might not
hold it. If so, I think *might* would make more sense here, like this:

* Context: User context.  May sleep.  Caller might hold mmap_lock.

WDYT?