linux-kernel - Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user

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Date:   Tue, 19 Jun 2018 18:57:41 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <john.hubbard@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()

On Tue, Jun 19, 2018 at 6:34 PM, John Hubbard <jhubbard@...dia.com> wrote:
> On 06/19/2018 06:24 PM, Dan Williams wrote:
>> On Tue, Jun 19, 2018 at 11:11 AM, John Hubbard <jhubbard@...dia.com> wrote:
>>> On 06/19/2018 03:41 AM, Jan Kara wrote:
>>>> On Tue 19-06-18 02:02:55, Matthew Wilcox wrote:
>>>>> On Tue, Jun 19, 2018 at 10:29:49AM +0200, Jan Kara wrote:
>> [..]
>>>> And then there's the aspect that both these approaches are a bit too
>>>> heavyweight for some get_user_pages_fast() users (e.g. direct IO) - Al Viro
>>>> had an idea to use page lock for that path but e.g. fs/direct-io.c would have
>>>> problems due to lock ordering constraints (filesystem ->get_block would
>>>> suddently get called with the page lock held). But we can probably leave
>>>> performance optimizations for phase two.
>>>
>>>
>>> So I assume that phase one would be to apply this approach only to
>>> get_user_pages_longterm. (Please let me know if that's wrong.)
>>
>> I think that's wrong, because get_user_pages_longterm() is only a
>> filesystem-dax avoidance mechanism, it's not trying to address all the
>> problems that Jan is talking about. I don't see any viable half-step
>> solutions.
>>
>
> OK, but in that case, I'm slightly confused by Jan's comment above, about leaving
> performance optimizations until phase two. Because that *is* a half-step approach:
> phase one, phase two.

No, sorry, I might be confusing things. The half step is leaving
truncate broken, or my strawman that only addressed unmap.

> Are you disagreeing with Jan, or are you suggesting "fix get_user_pages first, and
> leave get_user_pages_fast alone for now?"

I'm agreeing with Jan, we need to fix page_mkclean() and
try_to_unmap() without regressing truncate behavior.