linux-ext4 - Re: [RFC PATCH v2 15/19] mm/gup: Introduce vaddr_pin

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Date:   Mon, 12 Aug 2019 14:00:14 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Theodore Ts'o <tytso@....edu>, Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.com>, linux-xfs@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-ext4@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v2 15/19] mm/gup: Introduce vaddr_pin_pages()

On Fri, Aug 09, 2019 at 05:09:54PM -0700, John Hubbard wrote:
> On 8/9/19 3:58 PM, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > The addition of FOLL_LONGTERM has taken on additional meaning for CMA
> > pages.
> > 
> > In addition subsystems such as RDMA require new information to be passed
> > to the GUP interface to track file owning information.  As such a simple
> > FOLL_LONGTERM flag is no longer sufficient for these users to pin pages.
> > 
> > Introduce a new GUP like call which takes the newly introduced vaddr_pin
> > information.  Failure to pass the vaddr_pin object back to a vaddr_put*
> > call will result in a failure if pins were created on files during the
> > pin operation.
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > 
> > ---
> > Changes from list:
> > 	Change to vaddr_put_pages_dirty_lock
> > 	Change to vaddr_unpin_pages_dirty_lock
> > 
> >  include/linux/mm.h |  5 ++++
> >  mm/gup.c           | 59 ++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 64 insertions(+)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 657c947bda49..90c5802866df 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1603,6 +1603,11 @@ int account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc);
> >  int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc,
> >  			struct task_struct *task, bool bypass_rlim);
> >  
> > +long vaddr_pin_pages(unsigned long addr, unsigned long nr_pages,
> > +		     unsigned int gup_flags, struct page **pages,
> > +		     struct vaddr_pin *vaddr_pin);
> > +void vaddr_unpin_pages_dirty_lock(struct page **pages, unsigned long nr_pages,
> > +				  struct vaddr_pin *vaddr_pin, bool make_dirty);
> 
> Hi Ira,
> 
> OK, the API seems fine to me, anyway. :)
> 
> A bit more below...
> 
> >  bool mapping_inode_has_layout(struct vaddr_pin *vaddr_pin, struct page *page);
> >  
> >  /* Container for pinned pfns / pages */
> > diff --git a/mm/gup.c b/mm/gup.c
> > index eeaa0ddd08a6..6d23f70d7847 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2536,3 +2536,62 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> >  	return ret;
> >  }
> >  EXPORT_SYMBOL_GPL(get_user_pages_fast);
> > +
> > +/**
> > + * vaddr_pin_pages pin pages by virtual address and return the pages to the
> > + * user.
> > + *
> > + * @addr, start address
> 
> What's with the commas? I thought kernel-doc wants colons, like this, right?
> 
> @addr: start address

:-/  I don't know.

Fixed.

> 
> 
> > + * @nr_pages, number of pages to pin
> > + * @gup_flags, flags to use for the pin
> > + * @pages, array of pages returned
> > + * @vaddr_pin, initalized meta information this pin is to be associated
> > + * with.
> > + *
> > + * NOTE regarding vaddr_pin:
> > + *
> > + * Some callers can share pins via file descriptors to other processes.
> > + * Callers such as this should use the f_owner field of vaddr_pin to indicate
> > + * the file the fd points to.  All other callers should use the mm this pin is
> > + * being made against.  Usually "current->mm".
> > + *
> > + * Expects mmap_sem to be read locked.
> > + */
> > +long vaddr_pin_pages(unsigned long addr, unsigned long nr_pages,
> > +		     unsigned int gup_flags, struct page **pages,
> > +		     struct vaddr_pin *vaddr_pin)
> > +{
> > +	long ret;
> > +
> > +	gup_flags |= FOLL_LONGTERM;
> 
> 
> Is now the right time to introduce and use FOLL_PIN? If not, then I can always
> add it on top of this later, as part of gup-tracking patches. But you did point
> out that FOLL_LONGTERM is taking on additional meaning, and so maybe it's better
> to split that meaning up right from the start.
> 

At one point I wanted to (and had in my tree) a new flag but I went away from
it.  Prior to the discussion on mlock last week I did not think we needed it.
But I'm ok to add it back in.

I was not ignoring the idea for this RFC I just wanted to get this out there
for people to see.  I see that you threw out a couple of patches which add this
flag in.

FWIW, I think it would be good to differentiate between an indefinite pinned
page vs a referenced "gotten" page.

What you and I have been working on is the former.  So it would be easy to
change your refcounting patches to simply key off of FOLL_PIN.

Would you like me to add in your FOLL_PIN patches to this series?

> 
> > +
> > +	if (!vaddr_pin || (!vaddr_pin->mm && !vaddr_pin->f_owner))
> > +		return -EINVAL;
> > +
> > +	ret = __gup_longterm_locked(current,
> > +				    vaddr_pin->mm,
> > +				    addr, nr_pages,
> > +				    pages, NULL, gup_flags,
> > +				    vaddr_pin);
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(vaddr_pin_pages);
> > +
> > +/**
> > + * vaddr_unpin_pages_dirty_lock - counterpart to vaddr_pin_pages
> > + *
> > + * @pages, array of pages returned
> > + * @nr_pages, number of pages in pages
> > + * @vaddr_pin, same information passed to vaddr_pin_pages
> > + * @make_dirty: whether to mark the pages dirty
> > + *
> > + * The semantics are similar to put_user_pages_dirty_lock but a vaddr_pin used
> > + * in vaddr_pin_pages should be passed back into this call for propper
> 
> Typo:
                                                                   proper
Fixed.

> 
> > + * tracking.
> > + */
> > +void vaddr_unpin_pages_dirty_lock(struct page **pages, unsigned long nr_pages,
> > +				  struct vaddr_pin *vaddr_pin, bool make_dirty)
> > +{
> > +	__put_user_pages_dirty_lock(vaddr_pin, pages, nr_pages, make_dirty);
> > +}
> > +EXPORT_SYMBOL(vaddr_unpin_pages_dirty_lock);
> > 
> 
> OK, whew, I'm glad to see the updated _dirty_lock() API used here. :)

Yea this was pretty easy to change during the rebase.  Again I'm kind of
floating these quickly at this point.  So sorry about the nits...

Ira

> 
> thanks,
> -- 
> John Hubbard
> NVIDIA