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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>,
	John Hubbard <jhubbard@nvidia.com>,
	linux-rdma <linux-rdma@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Netdev <netdev@vger.kernel.org>,
	Mike Marciniszyn <mike.marciniszyn@intel.com>,
	Dennis Dalessandro <dennis.dalessandro@intel.com>,
	Doug Ledford <dledford@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 2/3] mm/gup: Introduce get_user_pages_fast_longterm()
Date: Mon, 11 Feb 2019 16:25:10 -0700	[thread overview]
Message-ID: <20190211232510.GP24692@ziepe.ca> (raw)
In-Reply-To: <CAPcyv4htDHmH7PVm_=HOWwRKtpcKTPSjrHPLqhwp2vhBUWL4-w@mail.gmail.com>

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

  parent reply	other threads:[~2019-02-11 23:25 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-11 20:16 [PATCH 0/3] Add gup fast + longterm and use it in HFI1 ira.weiny
2019-02-11 20:16 ` [PATCH 1/3] mm/gup: Change "write" parameter to flags ira.weiny
2019-02-11 20:16 ` [PATCH 2/3] mm/gup: Introduce get_user_pages_fast_longterm() ira.weiny
2019-02-11 20:39   ` Jason Gunthorpe
2019-02-11 21:13     ` John Hubbard
2019-02-11 21:26       ` Ira Weiny
2019-02-11 21:39         ` John Hubbard
2019-02-11 21:45           ` Dan Williams
2019-02-11 21:52           ` Ira Weiny
2019-02-11 22:01             ` John Hubbard
2019-02-11 22:06             ` Jason Gunthorpe
2019-02-11 22:55               ` Dan Williams
2019-02-11 23:04                 ` Weiny, Ira
2019-02-11 23:25                 ` Jason Gunthorpe [this message]
2019-02-12  0:08                   ` Ira Weiny
2019-02-11 20:16 ` [PATCH 3/3] IB/HFI1: Use new get_user_pages_fast_longterm() ira.weiny
2019-02-11 20:34 ` [PATCH 0/3] Add gup fast + longterm and use it in HFI1 Davidlohr Bueso
2019-02-11 20:47   ` Jason Gunthorpe
2019-02-11 21:42     ` Ira Weiny
2019-02-11 22:22       ` Jason Gunthorpe
2019-02-11 22:40         ` Weiny, Ira
2019-02-11 22:50           ` Jason Gunthorpe
2019-02-11 21:29   ` Ira Weiny
2019-02-11 20:40 ` Jason Gunthorpe
2019-02-11 21:14   ` Weiny, Ira
2019-02-11 22:23     ` Jason Gunthorpe
2019-02-13 23:04 ` [PATCH V2 0/7] Add FOLL_LONGTERM to GUP fast and use it ira.weiny
2019-02-13 23:04   ` [PATCH V2 1/7] mm/gup: Replace get_user_pages_longterm() with FOLL_LONGTERM ira.weiny
2019-02-13 23:04   ` [PATCH V2 2/7] mm/gup: Change write parameter to flags in fast walk ira.weiny
2019-02-13 23:04   ` [PATCH V2 3/7] mm/gup: Change GUP fast to use flags rather than a write 'bool' ira.weiny
2019-02-13 23:04   ` [PATCH V2 4/7] mm/gup: Add FOLL_LONGTERM capability to GUP fast ira.weiny
2019-02-13 23:04   ` [PATCH V2 5/7] IB/hfi1: Use the new FOLL_LONGTERM flag to get_user_pages_fast() ira.weiny
2019-02-13 23:04   ` [PATCH V2 6/7] IB/qib: " ira.weiny
2019-02-13 23:04   ` [PATCH V2 7/7] IB/mthca: " ira.weiny

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