linux-kernel - Re: [PATCH 2/2] mm/frame-vec: use FOLL

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Date:   Mon, 5 Oct 2020 14:35:31 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     John Hubbard <jhubbard@...dia.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

On Sun, Oct 04, 2020 at 01:20:31PM +0200, Daniel Vetter wrote:

> Yeah I think that works. I tried understanding gup.c code a bit more,
> and it looks like FOLL_LONGTERM only works for the pup_fast variant
> right now? All others seem to have this comment that it's somehow
> incompatible with FAULT_FLAG_ALLOW_RETRY and daxfs. But grepping
> around for that didn't show up anything, at least not nearby dax code.
> For my understanding of all this, what's the hiccup there?

IIRC it needs the VMA and various other flows can't return the vma
list becuse they unlock the mmap_sem will running

> For plans, I only started this for a bit of my own learning, but I
> think I'll respin with the following changes:
> - convert exynos and habanalabs to pin_user_pages_fast directly,
> instead of going through this frame-vector detour

+1

Jason