linux-kernel - Re: [PATCH 1/5] mm: Introduce mm_struct.has

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Message-ID: <8bc86452-3a7e-c435-014b-5acc21f591d2@nvidia.com>
Date:   Mon, 21 Sep 2020 17:01:19 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Peter Xu <peterx@...hat.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
CC:     Jason Gunthorpe <jgg@...pe.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Jann Horn" <jannh@...gle.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

On 9/21/20 4:53 PM, John Hubbard wrote:
> On 9/21/20 2:17 PM, Peter Xu wrote:
>> (Commit message collected from Jason Gunthorpe)
>>
>> Reduce the chance of false positive from page_maybe_dma_pinned() by keeping
> 
> Not yet, it doesn't. :)  More:
> 
>> track if the mm_struct has ever been used with pin_user_pages(). mm_structs
>> that have never been passed to pin_user_pages() cannot have a positive
>> page_maybe_dma_pinned() by definition. This allows cases that might drive up
>> the page ref_count to avoid any penalty from handling dma_pinned pages.
>>
>> Due to complexities with unpining this trivial version is a permanent sticky
>> bit, future work will be needed to make this a counter.
> 
> How about this instead:
> 
> Subsequent patches intend to reduce the chance of false positives from
> page_maybe_dma_pinned(), by also considering whether or not a page has
> even been part of an mm struct that has ever had pin_user_pages*()


arggh, correction: please make that:

     "...whether or not a page is part of an mm struct that...".

(Present tense.) Otherwise, people start wondering about the checkered past
of a page's past lives, and it badly distracts from the main point here. :)


thanks,
-- 
John Hubbard
NVIDIA