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From: Ira Weiny <ira.weiny@intel.com>
To: John Hubbard <jhubbard@nvidia.com>
Cc: john.hubbard@gmail.com,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Boaz Harrosh" <boaz@plexistor.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"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 3/3] net/xdp: convert put_page() to put_user_page*()
Date: Tue, 23 Jul 2019 11:06:13 -0700	[thread overview]
Message-ID: <20190723180612.GB29729@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <a4e9b293-11f8-6b3c-cf4d-308e3b32df34@nvidia.com>

On Mon, Jul 22, 2019 at 09:41:34PM -0700, John Hubbard wrote:
> On 7/22/19 5:25 PM, Ira Weiny wrote:
> > On Mon, Jul 22, 2019 at 03:34:15PM -0700, john.hubbard@gmail.com wrote:
> > > From: John Hubbard <jhubbard@nvidia.com>
> > > 
> > > For pages that were retained via get_user_pages*(), release those pages
> > > via the new put_user_page*() routines, instead of via put_page() or
> > > release_pages().
> > > 
> > > This is part a tree-wide conversion, as described in commit fc1d8e7cca2d
> > > ("mm: introduce put_user_page*(), placeholder versions").
> > > 
> > > Cc: Björn Töpel <bjorn.topel@intel.com>
> > > Cc: Magnus Karlsson <magnus.karlsson@intel.com>
> > > Cc: David S. Miller <davem@davemloft.net>
> > > Cc: netdev@vger.kernel.org
> > > Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> > > ---
> > >   net/xdp/xdp_umem.c | 9 +--------
> > >   1 file changed, 1 insertion(+), 8 deletions(-)
> > > 
> > > diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> > > index 83de74ca729a..0325a17915de 100644
> > > --- a/net/xdp/xdp_umem.c
> > > +++ b/net/xdp/xdp_umem.c
> > > @@ -166,14 +166,7 @@ void xdp_umem_clear_dev(struct xdp_umem *umem)
> > >   static void xdp_umem_unpin_pages(struct xdp_umem *umem)
> > >   {
> > > -	unsigned int i;
> > > -
> > > -	for (i = 0; i < umem->npgs; i++) {
> > > -		struct page *page = umem->pgs[i];
> > > -
> > > -		set_page_dirty_lock(page);
> > > -		put_page(page);
> > > -	}
> > > +	put_user_pages_dirty_lock(umem->pgs, umem->npgs);
> > 
> > What is the difference between this and
> > 
> > __put_user_pages(umem->pgs, umem->npgs, PUP_FLAGS_DIRTY_LOCK);
> > 
> > ?
> 
> No difference.
> 
> > 
> > I'm a bit concerned with adding another form of the same interface.  We should
> > either have 1 call with flags (enum in this case) or multiple calls.  Given the
> > previous discussion lets move in the direction of having the enum but don't
> > introduce another caller of the "old" interface.
> 
> I disagree that this is a "problem". There is no maintenance pitfall here; there
> are merely two ways to call the put_user_page*() API. Both are correct, and
> neither one will get you into trouble.
> 
> Not only that, but there is ample precedent for this approach in other
> kernel APIs.
> 
> > 
> > So I think on this patch NAK from me.
> > 
> > I also don't like having a __* call in the exported interface but there is a
> > __get_user_pages_fast() call so I guess there is precedent.  :-/
> > 
> 
> I thought about this carefully, and looked at other APIs. And I noticed that
> things like __get_user_pages*() are how it's often done:
> 
> * The leading underscores are often used for the more elaborate form of the
> call (as oppposed to decorating the core function name with "_flags", for
> example).
> 
> * There are often calls in which you can either call the simpler form, or the
> form with flags and additional options, and yes, you'll get the same result.
> 
> Obviously, this stuff is all subject to a certain amount of opinion, but I
> think I'm on really solid ground as far as precedent goes. So I'm pushing
> back on the NAK... :)

Fair enough...  However, we have discussed in the past how GUP can be a
confusing interface to use.

So I'd like to see it be more directed.  Only using the __put_user_pages()
version allows us to ID callers easier through a grep of PUP_FLAGS_DIRTY_LOCK
in addition to directing users to use that interface rather than having to read
the GUP code to figure out that the 2 calls above are equal.  It is not a huge
deal but...

Ira

> 
> thanks,
> -- 
> John Hubbard
> NVIDIA
> 

WARNING: multiple messages have this Message-ID
From: Ira Weiny <ira.weiny@intel.com>
To: John Hubbard <jhubbard@nvidia.com>
Cc: john.hubbard@gmail.com,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Boaz Harrosh" <boaz@plexistor.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"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>
Subject: Re: [PATCH 3/3] net/xdp: convert put_page() to put_user_page*()
Date: Tue, 23 Jul 2019 11:06:13 -0700	[thread overview]
Message-ID: <20190723180612.GB29729@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <a4e9b293-11f8-6b3c-cf4d-308e3b32df34@nvidia.com>

On Mon, Jul 22, 2019 at 09:41:34PM -0700, John Hubbard wrote:
> On 7/22/19 5:25 PM, Ira Weiny wrote:
> > On Mon, Jul 22, 2019 at 03:34:15PM -0700, john.hubbard@gmail.com wrote:
> > > From: John Hubbard <jhubbard@nvidia.com>
> > > 
> > > For pages that were retained via get_user_pages*(), release those pages
> > > via the new put_user_page*() routines, instead of via put_page() or
> > > release_pages().
> > > 
> > > This is part a tree-wide conversion, as described in commit fc1d8e7cca2d
> > > ("mm: introduce put_user_page*(), placeholder versions").
> > > 
> > > Cc: Björn Töpel <bjorn.topel@intel.com>
> > > Cc: Magnus Karlsson <magnus.karlsson@intel.com>
> > > Cc: David S. Miller <davem@davemloft.net>
> > > Cc: netdev@vger.kernel.org
> > > Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> > > ---
> > >   net/xdp/xdp_umem.c | 9 +--------
> > >   1 file changed, 1 insertion(+), 8 deletions(-)
> > > 
> > > diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> > > index 83de74ca729a..0325a17915de 100644
> > > --- a/net/xdp/xdp_umem.c
> > > +++ b/net/xdp/xdp_umem.c
> > > @@ -166,14 +166,7 @@ void xdp_umem_clear_dev(struct xdp_umem *umem)
> > >   static void xdp_umem_unpin_pages(struct xdp_umem *umem)
> > >   {
> > > -	unsigned int i;
> > > -
> > > -	for (i = 0; i < umem->npgs; i++) {
> > > -		struct page *page = umem->pgs[i];
> > > -
> > > -		set_page_dirty_lock(page);
> > > -		put_page(page);
> > > -	}
> > > +	put_user_pages_dirty_lock(umem->pgs, umem->npgs);
> > 
> > What is the difference between this and
> > 
> > __put_user_pages(umem->pgs, umem->npgs, PUP_FLAGS_DIRTY_LOCK);
> > 
> > ?
> 
> No difference.
> 
> > 
> > I'm a bit concerned with adding another form of the same interface.  We should
> > either have 1 call with flags (enum in this case) or multiple calls.  Given the
> > previous discussion lets move in the direction of having the enum but don't
> > introduce another caller of the "old" interface.
> 
> I disagree that this is a "problem". There is no maintenance pitfall here; there
> are merely two ways to call the put_user_page*() API. Both are correct, and
> neither one will get you into trouble.
> 
> Not only that, but there is ample precedent for this approach in other
> kernel APIs.
> 
> > 
> > So I think on this patch NAK from me.
> > 
> > I also don't like having a __* call in the exported interface but there is a
> > __get_user_pages_fast() call so I guess there is precedent.  :-/
> > 
> 
> I thought about this carefully, and looked at other APIs. And I noticed that
> things like __get_user_pages*() are how it's often done:
> 
> * The leading underscores are often used for the more elaborate form of the
> call (as oppposed to decorating the core function name with "_flags", for
> example).
> 
> * There are often calls in which you can either call the simpler form, or the
> form with flags and additional options, and yes, you'll get the same result.
> 
> Obviously, this stuff is all subject to a certain amount of opinion, but I
> think I'm on really solid ground as far as precedent goes. So I'm pushing
> back on the NAK... :)

Fair enough...  However, we have discussed in the past how GUP can be a
confusing interface to use.

So I'd like to see it be more directed.  Only using the __put_user_pages()
version allows us to ID callers easier through a grep of PUP_FLAGS_DIRTY_LOCK
in addition to directing users to use that interface rather than having to read
the GUP code to figure out that the 2 calls above are equal.  It is not a huge
deal but...

Ira

> 
> thanks,
> -- 
> John Hubbard
> NVIDIA
> 

  parent reply	other threads:[~2019-07-23 18:06 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-22 22:34 [PATCH 0/3] introduce __put_user_pages(), convert a few call sites john.hubbard
2019-07-22 22:34 ` john.hubbard
2019-07-22 22:34 ` [PATCH 1/3] mm/gup: introduce __put_user_pages() john.hubbard
2019-07-22 22:34   ` john.hubbard
2019-07-23  5:53   ` Christoph Hellwig
2019-07-23  6:33     ` John Hubbard
2019-07-23  6:33       ` John Hubbard
2019-07-23 15:36       ` Christoph Hellwig
2019-07-23 15:36         ` Christoph Hellwig
2019-07-22 22:34 ` [PATCH 2/3] drivers/gpu/drm/via: convert put_page() to put_user_page*() john.hubbard
2019-07-22 22:34   ` john.hubbard
2019-07-22 22:34 ` [PATCH 3/3] net/xdp: " john.hubbard
2019-07-22 22:34   ` john.hubbard
2019-07-23  0:25   ` Ira Weiny
2019-07-23  0:25     ` Ira Weiny
2019-07-23  4:41     ` John Hubbard
2019-07-23  4:41       ` John Hubbard
2019-07-23 12:47       ` Jason Gunthorpe
2019-07-23 12:47         ` Jason Gunthorpe
2019-07-23 18:06       ` Ira Weiny [this message]
2019-07-23 18:06         ` Ira Weiny
2019-07-23 23:24         ` John Hubbard
2019-07-23 23:24           ` John Hubbard

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