linux-kernel - Re: [PATCH 1/2] mm: introduce put_user

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Date:   Tue, 29 Jan 2019 18:21:56 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>
CC:     Jerome Glisse <jglisse@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        Dan Williams <dan.j.williams@...el.com>,
        John Hubbard <john.hubbard@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, <tom@...pey.com>,
        Al Viro <viro@...iv.linux.org.uk>, <benve@...co.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Michal Hocko <mhocko@...nel.org>, <mike.marciniszyn@...el.com>,
        <rcampbell@...dia.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

On 1/29/19 2:12 AM, Jan Kara wrote:
> On Mon 28-01-19 22:41:41, John Hubbard wrote:
[...]
>> Here is the case I'm wondering about:
>>
>> thread A                             thread B
>> --------                             --------
>>                                      gup_fast
>> page_mkclean
>>     is page gup-pinned?(no)
>>                                          page_cache_get_speculative
>>                                              (gup-pins the page here)
>>                                          check pte_val unchanged (yes)
>>        set_pte_at()
>>
>> ...and now thread A has created a read-only PTE, after gup_fast walked
>> the page tables and found a writeable entry. And so far, thread A has
>> not seen that the page is pinned.
>>
>> What am I missing here? The above seems like a problem even before we
>> change anything.
> 
> Your implementation of page_mkclean() is wrong :) It needs to first call
> set_pte_at() and only after that ask "is page gup pinned?". In fact,
> page_mkclean() probably has no bussiness in checking for page pins
> whatsoever. It is clear_page_dirty_for_io() that cares, so that should
> check for page pins after page_mkclean() has returned.
> 

Perfect, that was the missing piece for me: page_mkclean() internally doesn't
need the consistent view, just the caller does. The whole situation with
two distinct lock-free algorithms going on here actually seems clear at last. :)

Thanks (also to Jerome) for explaining this!

thanks,
-- 
John Hubbard
NVIDIA