linux-kernel - Re: [PATCH 1/4] mm: Trial do_wp

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Message-ID: <CAFCwf10WEwrZzD7nWkyaZYnnYtZs0aWYwBniEPrb_xdgiQiggw@mail.gmail.com>
Date:   Wed, 16 Feb 2022 21:20:49 +0200
From:   Oded Gabbay <ogabbay@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        Kirill Shutemov <kirill@...temov.name>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Xu <peterx@...hat.com>, dave@...1.net
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

On Wed, Feb 16, 2022 at 9:04 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> [ Added David Hildenbrand to the participants. David, see
>
>     https://bugzilla.kernel.org/show_bug.cgi?id=215616
>
>   for details ]
>
> On Wed, Feb 16, 2022 at 8:59 AM Oded Gabbay <oded.gabbay@...il.com> wrote:
> >
> > All the details are in the bug, but the bottom line is that somehow,
> > this patch causes corruption when the numa balancing feature is
> > enabled AND we don't use process affinity AND we use GUP to pin pages
> > so our accelerator can DMA to/from system memory.
>
> Hmm. I thought all the remaining issues were related to THP - and
> David Hildenbrand had a series to fix those up.
>
> The fact that it also shows up with numa balancing is a bit
> unfortunate, because I think that means that that patch series may not
> have caught that case.
>
> That said - what does "we use GUP to pin pages" mean? Does it actually
> use the pinning logic, or just regular old GUP?
>
> I'm assuming this is just the existing pin_user_pages_fast() (ie a
> proper pin) in drivers/misc/habanalabs/common/memory.c. But I wanted
> to confirm that it's not some other situation.
>
>                    Linus
>
[Adding dave hansen as we chatted about it in irc]

It uses the pinning logic, simply calling pin_user_pages_fast with the
relevant gup flags to pin the userspace memory so we can after that
dma map it and give the bus address to the h/w.

And afaik, the gup flags combination we use
(FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM) is the correct combination,
at least according to the last time it was discussed with Daniel,
Jason and other people.

Oded