linux-kernel - Re: [PATCH 2/3] mm/gup: Introduce get_user_pages_fast

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Date:   Mon, 11 Feb 2019 16:25:10 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Davidlohr Bueso <dave@...olabs.net>,
        Netdev <netdev@...r.kernel.org>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 2/3] mm/gup: Introduce get_user_pages_fast_longterm()

On Mon, Feb 11, 2019 at 02:55:10PM -0800, Dan Williams wrote:

> > I also wonder if someone should think about making fast into a flag
> > too..
> >
> > But I'm not sure when fast should be used vs when it shouldn't :(
> 
> Effectively fast should always be used just in case the user cares
> about performance. It's just that it may fail and need to fall back to
> requiring the vma.

But the fall back / slow path is hidden inside the API, so when should
the caller care? 

ie when should the caller care to use gup_fast vs gup_unlocked? (the
comments say they are the same, but this seems to be a mistake)

Based on some of the comments in the code it looks like this API is
trying to convert itself into:

long get_user_pages_locked(struct task_struct *tsk, struct mm_struct *mm,
                           unsigned long start, unsigned long nr_pages,
			   unsigned int gup_flags, struct page **pages,
			   struct vm_area_struct **vmas, bool *locked)

long get_user_pages_unlocked(struct task_struct *tsk, struct mm_struct *mm,
                             unsigned long start, unsigned long nr_pages,
			     unsigned int gup_flags, struct page **pages)

(and maybe a FOLL_FAST if there is some reason we have _fast and
_unlocked)

The reason I ask, is that if there is no reason for fast vs unlocked
then maybe Ira should convert HFI to use gup_unlocked and move the
'fast' code into unlocked?

ie move incrementally closer to the desired end-state here.

Jason