linux-kernel - Re: [PATCH RESEND] drm/via: use get_user_pages

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Message-ID: <CAA5enKa4Asp4qSHkeV3saLZrhOMf2DJ9vuiwTDo1t5t54z4sTQ@mail.gmail.com>
Date:   Tue, 28 Feb 2017 20:28:08 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel@...ts.freedesktop.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH RESEND] drm/via: use get_user_pages_unlocked()

On 28 February 2017 at 19:35, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, Feb 28, 2017 at 10:01:10AM +0100, Daniel Vetter wrote:
>
>> > +   ret = get_user_pages_unlocked((unsigned long)xfer->mem_addr,
>> > +                   vsg->num_pages, vsg->pages,
>> > +                   (vsg->direction == DMA_FROM_DEVICE) ? FOLL_WRITE : 0);
>
> Umm...  Why not
>         ret = get_user_pages_fast((unsigned long)xfer->mem_addr,
>                         vsg->num_pages,
>                         vsg->direction == DMA_FROM_DEVICE,
>                         vsg->pages);
>
> IOW, do you really need a warranty that ->mmap_sem will be grabbed and
> released?

Daniel will be better placed to answer in this specific case, but more
generally is there any reason why we can't just use
get_user_pages_fast() in all such cases? These patches were simply a
mechanical/cautious replacement for code that is more or less exactly
equivalent but if this would make sense perhaps it'd be worth using
gup_fast() where possible?

-- 
Lorenzo Stoakes
https://ljs.io