Re: [PATCH 1/3] mm/gup: introduce __put_user_pages()

From: John Hubbard <jhubbard@nvidia.com>
To: Christoph Hellwig <hch@lst.de>, <john.hubbard@gmail.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Boaz Harrosh" <boaz@plexistor.com>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Dave Chinner" <david@fromorbit.com>,
	"David Airlie" <airlied@linux.ie>,
	"David S . Miller" <davem@davemloft.net>,
	"Ilya Dryomov" <idryomov@gmail.com>, "Jan Kara" <jack@suse.cz>,
	"Jason Gunthorpe" <jgg@ziepe.ca>, "Jens Axboe" <axboe@kernel.dk>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Johannes Thumshirn" <jthumshirn@suse.de>,
	"Magnus Karlsson" <magnus.karlsson@intel.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Miklos Szeredi" <miklos@szeredi.hu>,
	"Ming Lei" <ming.lei@redhat.com>, "Sage Weil" <sage@redhat.com>,
	"Santosh Shilimkar" <santosh.shilimkar@oracle.com>,
	"Yan Zheng" <zyan@redhat.com>,
	netdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
	linux-mm@kvack.org, linux-rdma@vger.kernel.org,
	bpf@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/3] mm/gup: introduce __put_user_pages()
Date: Mon, 22 Jul 2019 23:33:32 -0700	[thread overview]
Message-ID: <8ab4899c-ec12-a713-cac2-d951fff2a347@nvidia.com> (raw)
In-Reply-To: <20190723055359.GC17148@lst.de>

On 7/22/19 10:53 PM, Christoph Hellwig wrote:
> On Mon, Jul 22, 2019 at 03:34:13PM -0700, john.hubbard@gmail.com wrote:
>> +enum pup_flags_t {
>> +	PUP_FLAGS_CLEAN		= 0,
>> +	PUP_FLAGS_DIRTY		= 1,
>> +	PUP_FLAGS_LOCK		= 2,
>> +	PUP_FLAGS_DIRTY_LOCK	= 3,
>> +};
> 
> Well, the enum defeats the ease of just being able to pass a boolean
> expression to the function, which would simplify a lot of the caller,
> so if we need to support the !locked version I'd rather see that as
> a separate helper.
> 
> But do we actually have callers where not using the _lock version is
> not a bug?  set_page_dirty makes sense in the context of a file systems
> that have a reference to the inode the page hangs off, but that is
> (almost?) never the case for get_user_pages.
> 

I'm seeing about 18 places where set_page_dirty() is used, in the call site
conversions so far, and about 20 places where set_page_dirty_lock() is
used. So without knowing how many of the former (if any) represent bugs,
you can see why the proposal here supports both DIRTY and DIRTY_LOCK.

Anyway, yes, I could change it, based on your estimation that most of the 
set_page_dirty() calls really should be set_page_dirty_lock().
In that case, we would end up with approximately the following:

/* Here, "dirty" really means, "call set_page_dirty_lock()": */
void __put_user_pages(struct page **pages, unsigned long npages,
		      bool dirty);

/* Here, "dirty" really means, "call set_page_dirty()": */
void __put_user_pages_unlocked(struct page **pages, unsigned long npages,
			       bool dirty);

?

thanks,
-- 
John Hubbard
NVIDIA