linux-kernel - Re: [PATCH v4 1/1] mm: introduce put_user

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Message-ID: <f9195df4-66ca-95f6-874e-d19cd775794d@talpey.com>
Date:   Tue, 19 Mar 2019 15:43:44 -0500
From:   Tom Talpey <tom@...pey.com>
To:     Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Jerome Glisse <jglisse@...hat.com>, john.hubbard@...il.com,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Benvenuti <benve@...co.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder
 versions

On 3/19/2019 4:03 AM, Ira Weiny wrote:
> On Tue, Mar 19, 2019 at 04:36:44PM +0100, Jan Kara wrote:
>> On Tue 19-03-19 17:29:18, Kirill A. Shutemov wrote:
>>> On Tue, Mar 19, 2019 at 10:14:16AM -0400, Jerome Glisse wrote:
>>>> On Tue, Mar 19, 2019 at 09:47:24AM -0400, Jerome Glisse wrote:
>>>>> On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote:
>>>>>> On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@...il.com wrote:
>>>>>>> From: John Hubbard <jhubbard@...dia.com>
>>>>>
>>>>> [...]
>>>>>
>>>>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>>>>> index f84e22685aaa..37085b8163b1 100644
>>>>>>> --- a/mm/gup.c
>>>>>>> +++ b/mm/gup.c
>>>>>>> @@ -28,6 +28,88 @@ struct follow_page_context {
>>>>>>>   	unsigned int page_mask;
>>>>>>>   };
>>>>>>>   
>>>>>>> +typedef int (*set_dirty_func_t)(struct page *page);
>>>>>>> +
>>>>>>> +static void __put_user_pages_dirty(struct page **pages,
>>>>>>> +				   unsigned long npages,
>>>>>>> +				   set_dirty_func_t sdf)
>>>>>>> +{
>>>>>>> +	unsigned long index;
>>>>>>> +
>>>>>>> +	for (index = 0; index < npages; index++) {
>>>>>>> +		struct page *page = compound_head(pages[index]);
>>>>>>> +
>>>>>>> +		if (!PageDirty(page))
>>>>>>> +			sdf(page);
>>>>>>
>>>>>> How is this safe? What prevents the page to be cleared under you?
>>>>>>
>>>>>> If it's safe to race clear_page_dirty*() it has to be stated explicitly
>>>>>> with a reason why. It's not very clear to me as it is.
>>>>>
>>>>> The PageDirty() optimization above is fine to race with clear the
>>>>> page flag as it means it is racing after a page_mkclean() and the
>>>>> GUP user is done with the page so page is about to be write back
>>>>> ie if (!PageDirty(page)) see the page as dirty and skip the sdf()
>>>>> call while a split second after TestClearPageDirty() happens then
>>>>> it means the racing clear is about to write back the page so all
>>>>> is fine (the page was dirty and it is being clear for write back).
>>>>>
>>>>> If it does call the sdf() while racing with write back then we
>>>>> just redirtied the page just like clear_page_dirty_for_io() would
>>>>> do if page_mkclean() failed so nothing harmful will come of that
>>>>> neither. Page stays dirty despite write back it just means that
>>>>> the page might be write back twice in a row.
>>>>
>>>> Forgot to mention one thing, we had a discussion with Andrea and Jan
>>>> about set_page_dirty() and Andrea had the good idea of maybe doing
>>>> the set_page_dirty() at GUP time (when GUP with write) not when the
>>>> GUP user calls put_page(). We can do that by setting the dirty bit
>>>> in the pte for instance. They are few bonus of doing things that way:
>>>>      - amortize the cost of calling set_page_dirty() (ie one call for
>>>>        GUP and page_mkclean()
>>>>      - it is always safe to do so at GUP time (ie the pte has write
>>>>        permission and thus the page is in correct state)
>>>>      - safe from truncate race
>>>>      - no need to ever lock the page
>>>>
>>>> Extra bonus from my point of view, it simplify thing for my generic
>>>> page protection patchset (KSM for file back page).
>>>>
>>>> So maybe we should explore that ? It would also be a lot less code.
>>>
>>> Yes, please. It sounds more sensible to me to dirty the page on get, not
>>> on put.
>>
>> I fully agree this is a desirable final state of affairs.
> 
> I'm glad to see this presented because it has crossed my mind more than once
> that effectively a GUP pinned page should be considered "dirty" at all times
> until the pin is removed.  This is especially true in the RDMA case.

But, what if the RDMA registration is readonly? That's not uncommon, and
marking dirty unconditonally would add needless overhead to such pages.

Tom.