linux-kernel - Re: [PATCH 1/4] mm: add new mmgrab() helper

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Date:   Fri, 16 Dec 2016 12:14:40 +0100
From:   Vegard Nossum <vegard.nossum@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Rik van Riel <riel@...hat.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/4] mm: add new mmgrab() helper

On 12/16/2016 10:56 AM, Peter Zijlstra wrote:
> On Fri, Dec 16, 2016 at 09:21:59AM +0100, Vegard Nossum wrote:
>> Apart from adding the helper function itself, the rest of the kernel is
>> converted mechanically using:
>>
>>   git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)->mm_count);/mmgrab\(\1\);/'
>>   git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)\.mm_count);/mmgrab\(\&\1\);/'
>>
>> This is needed for a later patch that hooks into the helper, but might be
>> a worthwhile cleanup on its own.
>
> Given the desire to replace all refcounting with a specific refcount
> type, this seems to make sense.
>
> FYI: http://www.openwall.com/lists/kernel-hardening/2016/12/07/8

If we're going that way eventually (replacing all reference counting
things with a generic interface), I wonder if we shouldn't consider a
generic mechanism for reference counting debugging too.

We could wrap all the 'type *' + 'type_ref' occurrences in a struct, so
that with debugging it boils down to just a pointer (like we have now):

struct ref {
     void *ptr;
#ifdef CONFIG_REF_DEBUG
     /* list_entry, pid, stacktrace, etc. */
#endif
};

Instead of calling refcount_inc() in most of the kernel code, that would
be considered a low-level detail and you'd have the main interface be
something like:

void ref_acquire(refcount_t *count, struct ref *old, struct ref *new)
{
     refcount_inc(&count);
     new->ptr = old->ptr;
#ifdef CONFIG_REF_DEBUG
     /* extra code for debugging case */
#endif
}

So if you had old code that did (for example):

struct task_struct {
     struct mm_struct *mm;
     ...
};

int proc_pid_cmdline_read(struct task_struct *task)
{
     struct mm_struct *mm;

     task_lock(task);
     mm = task->mm;
     atomic_inc(&mm->mm_users);
     task_unlock(task);

     ...

     mmput(mm);
}

you'd instead have:

struct task_struct {
     struct ref mm;
};

int proc_pid_cmdline_read(struct task_struct *task)
{
     REF(mm);

     task_lock(task);
     ref_acquire(&mm->mm_users, &task->mm, &mm)
     task_unlock(task);

     ...

     ref_release(&mm->mm_users, &mm);
}

Of course you'd define a 'struct ref' per type using a macro or
something to keep it type safe (maybe even wrap the counter itself in
there, e.g. mm_users in the example above, so you wouldn't have to pass
it explicitly).

Functions that don't touch reference counts (because the caller holds
one) can just take a plain pointer as usual.

In the example above, you could also have ref_release() set mm->ptr =
NULL; as the pointer should not be considered usable after it has been
released anyway for added safety/debugability.

Best of both worlds?


Vegard