linux-kernel - Re: [PATCH] binder_alloc: Add missing mmap

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Message-ID: <YvLLQ/qasEJffdnn@google.com>
Date:   Tue, 9 Aug 2022 21:01:55 +0000
From:   Carlos Llamas <cmllamas@...gle.com>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        syzbot <syzbot+a7b60a176ec13cafb793@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        "syzkaller-bugs@...glegroups.com" <syzkaller-bugs@...glegroups.com>,
        Minchan Kim <minchan@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Martijn Coenen <maco@...roid.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Todd Kjos <tkjos@...roid.com>,
        Matthew Wilcox <willy@...radead.org>,
        Arve Hjønnevåg <arve@...roid.com>
Subject: Re: [PATCH] binder_alloc: Add missing mmap_lock calls when using the
 VMA

On Tue, Aug 09, 2022 at 07:02:17PM +0000, Liam Howlett wrote:
> > 
> > do we need to hold on to the lock while we loop through the pages here?
> 
> I think we do?  Holding this lock will ensure the pages don't go away, I
> believe (looking at mm/rmap.c comments on locking at the top)?
> 
> In any case, this function is called from print_binder_proc_stats()
> which looks to be a debugfs/debugging call so I thought safer would be
> better than faster and with a potential race.

The pages are protected by alloc->mutex, so you could immediately
release the mmap lock after binder_alloc_get_vma() call. I agree this
is a debugging call so it would be nice to reduce contention.