Re: [RFC PATCH 00/64] mm: towards parallel address space operations

[Davidlohr Bueso]

On Mon, 05 Feb 2018, Laurent Dufour wrote:

> On 05/02/2018 02:26, Davidlohr Bueso wrote:
> > From: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> >
> > Hi,
> >
> > This patchset is a new version of both the range locking machinery as well
> > as a full mmap_sem conversion that makes use of it -- as the worst case
> > scenario as all mmap_sem calls are converted to a full range mmap_lock
> > equivalent. As such, while there is no improvement of concurrency perse,
> > these changes aim at adding the machinery to permit this in the future.
>
> Despite the massive rebase, what are the changes in this series compared to
> the one I sent in last May - you silently based on, by the way :
> https://lkml.org/lkml/2017/5/24/409

Hardly, but yes I meant to reference that. It ended up being easier to just
do a from scratch version. I haven't done a comparison, but at first I thought
you missed gup users (now not so much), this patchset allows testing on more
archs (see below), we remove the trylock in vm_insert_page(), etc.

> > Direct users of the mm->mmap_sem can be classified as those that (1) acquire
> > and release the lock within the same context, and (2) those who directly
> > manipulate the mmap_sem down the callchain. For example:
> >
> > (1)  down_read(&mm->mmap_sem);
> >      /* do something */
> >      /* nobody down the chain uses mmap_sem directly */
> >      up_read(&mm->mmap_sem);
> >
> > (2a)  down_read(&mm->mmap_sem);
> >       /* do something that retuns mmap_sem unlocked */
> >       fn(mm, &locked);
> >       if (locked)
> >         up_read(&mm->mmap_sem);
> >
> > (2b)  down_read(&mm->mmap_sem);
> >       /* do something that in between released and reacquired mmap_sem */
> >       fn(mm);
> >       up_read(&mm->mmap_sem);
>
> Unfortunately, there are also indirect users which rely on the mmap_sem
> locking to protect their data. For the first step using a full range this
> doesn't matter, but when refining the range, these one would be the most
> critical ones as they would have to be reworked to take the range in account.

Of course. The value I see in this patchset is that we can determine whether or
not we move forward based on the worst case scenario numbers.

> > Testing: I have setup an mmtests config file with all the workloads described:
> > http://linux-scalability.org/mmtests-config
>
> Is this link still valid, I can't reach it ?

Sorry, that should have been:

https://linux-scalability.org/range-mmap_lock/mmtests-config

Thanks,
Davidlohr