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Message-ID: <20190211232510.GP24692@ziepe.ca> 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
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