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

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Message-ID: <9be3c203-6e44-6b9d-2331-afbcc269d0ff@nvidia.com>
Date:   Tue, 15 Jan 2019 13:39:17 -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 1/15/19 12:34 AM, Jan Kara wrote:
> On Mon 14-01-19 11:09:20, John Hubbard wrote:
>> On 1/14/19 9:21 AM, Jerome Glisse wrote:
>>>>
[...]
> 
>> For example, the following already survives a basic boot to graphics mode.
>> It requires a bunch of callsite conversions, and a page flag (neither of which
>> is shown here), and may also have "a few" gross conceptual errors, but take a 
>> peek:
> 
> Thanks for writing this down! Some comments inline.
> 

I appreciate your taking a look at this, Jan. I'm still pretty new to gup.c, 
so it's really good to get an early review.


>> +/*
>> + * Manages the PG_gup_pinned flag.
>> + *
>> + * Note that page->_mapcount counting part of managing that flag, because the
>> + * _mapcount is used to determine if PG_gup_pinned can be cleared, in
>> + * page_mkclean().
>> + */
>> +static void track_gup_page(struct page *page)
>> +{
>> +	page = compound_head(page);
>> +
>> +	lock_page(page);
>> +
>> +	wait_on_page_writeback(page);
> 
> ^^ I'd use wait_for_stable_page() here. That is the standard waiting
> mechanism to use before you allow page modification.

OK, will do. In fact, I initially wanted to use wait_for_stable_page(), but 
hesitated when I saw that it won't necessarily do wait_on_page_writeback(), 
and I then I also remembered Dave Chinner recently mentioned that the policy
decision needed some thought in the future (maybe something about block 
device vs. filesystem policy):

void wait_for_stable_page(struct page *page)
{
	if (bdi_cap_stable_pages_required(inode_to_bdi(page->mapping->host)))
		wait_on_page_writeback(page);
}

...but like you say, it's the standard way that fs does this, so we should
just use it.

> 
>> +
>> +	atomic_inc(&page->_mapcount);
>> +	SetPageGupPinned(page);
>> +
>> +	unlock_page(page);
>> +}
>> +
>> +/*
>> + * A variant of track_gup_page() that returns -EBUSY, instead of waiting.
>> + */
>> +static int track_gup_page_atomic(struct page *page)
>> +{
>> +	page = compound_head(page);
>> +
>> +	if (PageWriteback(page) || !trylock_page(page))
>> +		return -EBUSY;
>> +
>> +	if (PageWriteback(page)) {
>> +		unlock_page(page);
>> +		return -EBUSY;
>> +	}
> 
> Here you'd need some helper that would return whether
> wait_for_stable_page() is going to wait. Like would_wait_for_stable_page()
> but maybe you can come up with a better name.

Yes, in order to wait_for_stable_page(), that seems necessary, I agree.


thanks,
-- 
John Hubbard
NVIDIA