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From: John Hubbard <jhubbard@nvidia.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Dave Chinner" <david@fromorbit.com>,
	"David Airlie" <airlied@linux.ie>,
	"David S . Miller" <davem@davemloft.net>,
	"Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>,
	"Jason Gunthorpe" <jgg@ziepe.ca>, "Jens Axboe" <axboe@kernel.dk>,
	"Jonathan Corbet" <corbet@lwn.net>,
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	"Mike Rapoport" <rppt@linux.ibm.com>
Subject: Re: [PATCH v12 11/22] mm/gup: introduce pin_user_pages*() and FOLL_PIN
Date: Wed, 15 Jan 2020 13:34:16 -0800
Message-ID: <1a0ee1db-5528-86a8-0713-3d820fbdf4ad@nvidia.com> (raw)
In-Reply-To: <20200115153020.GF19546@infradead.org>

On 1/15/20 7:30 AM, Christoph Hellwig wrote:
> On Tue, Jan 07, 2020 at 02:45:47PM -0800, John Hubbard wrote:
>> Introduce pin_user_pages*() variations of get_user_pages*() calls,
>> and also pin_longterm_pages*() variations.
>>
>> For now, these are placeholder calls, until the various call sites
>> are converted to use the correct get_user_pages*() or
>> pin_user_pages*() API.
> 
> What do the pure placeholders buy us?  The API itself looks ok,
> but until it actually is properly implemented it doesn't help at
> all, and we've had all kinds of bad experiences with these sorts
> of stub APIs.
> 

Hi Christoph,

Absolutely agreed, and in fact, after spending some time in this area I 
am getting a much better understanding of just how problematic "this will 
be used soon" APIs really are. However, this is not quite that case.

The following things make this different from a "pure placeholder" API:

1) These APIs are all exercised in the following patches in this series, 
unless I've overlooked one, and

2) The pages are actually tracked in the very next patch that I want to
post. That patch was posted as part of the v11 series [1], but 
Leon Romanovsky reported a problem with it, and so I'm going to add in
the ability to handle larger "pin" refcounts for the huge page cases.

Meanwhile, I wanted to get these long-simmering simpler preparatory
patches submitted, because it's clear that the API is unaffected by the
huge page refcount fix. (That fix will likely use the second struct page of
the compound page, to count up higher.)


[1] https://lore.kernel.org/r/20191216222537.491123-24-jhubbard@nvidia.com  
    [PATCH v11 23/25] mm/gup: track FOLL_PIN pages

thanks,
-- 
John Hubbard
NVIDIA

  reply index

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-07 22:45 [PATCH v12 00/22] mm/gup: prereqs to track dma-pinned pages: FOLL_PIN John Hubbard
2020-01-07 22:45 ` [PATCH v12 01/22] mm/gup: factor out duplicate code from four routines John Hubbard
2020-01-07 22:45 ` [PATCH v12 02/22] mm/gup: move try_get_compound_head() to top, fix minor issues John Hubbard
2020-01-07 22:45 ` [PATCH v12 03/22] mm: Cleanup __put_devmap_managed_page() vs ->page_free() John Hubbard
2020-01-07 22:45 ` [PATCH v12 04/22] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages John Hubbard
2020-01-15 15:23   ` Christoph Hellwig
2020-01-15 21:19     ` John Hubbard
2020-01-16  9:37       ` Christoph Hellwig
2020-01-16 20:30         ` John Hubbard
2020-01-07 22:45 ` [PATCH v12 05/22] goldish_pipe: rename local pin_user_pages() routine John Hubbard
2020-01-15 15:23   ` Christoph Hellwig
2020-01-07 22:45 ` [PATCH v12 06/22] mm: fix get_user_pages_remote()'s handling of FOLL_LONGTERM John Hubbard
2020-01-15 15:24   ` Christoph Hellwig
2020-01-07 22:45 ` [PATCH v12 07/22] vfio: fix FOLL_LONGTERM use, simplify get_user_pages_remote() call John Hubbard
2020-01-07 22:45 ` [PATCH v12 08/22] mm/gup: allow FOLL_FORCE for get_user_pages_fast() John Hubbard
2020-01-15 15:25   ` Christoph Hellwig
2020-01-07 22:45 ` [PATCH v12 09/22] IB/umem: use get_user_pages_fast() to pin DMA pages John Hubbard
2020-01-07 22:45 ` [PATCH v12 10/22] media/v4l2-core: set pages dirty upon releasing DMA buffers John Hubbard
2020-01-07 22:45 ` [PATCH v12 11/22] mm/gup: introduce pin_user_pages*() and FOLL_PIN John Hubbard
2020-01-15 15:30   ` Christoph Hellwig
2020-01-15 21:34     ` John Hubbard [this message]
2020-01-07 22:45 ` [PATCH v12 12/22] goldish_pipe: convert to pin_user_pages() and put_user_page() John Hubbard
2020-01-07 22:45 ` [PATCH v12 13/22] IB/{core,hw,umem}: set FOLL_PIN via pin_user_pages*(), fix up ODP John Hubbard
2020-01-07 22:45 ` [PATCH v12 14/22] mm/process_vm_access: set FOLL_PIN via pin_user_pages_remote() John Hubbard
2020-01-07 22:45 ` [PATCH v12 15/22] drm/via: set FOLL_PIN via pin_user_pages_fast() John Hubbard
2020-01-07 22:45 ` [PATCH v12 16/22] fs/io_uring: set FOLL_PIN via pin_user_pages() John Hubbard
2020-01-07 22:45 ` [PATCH v12 17/22] net/xdp: " John Hubbard
2020-01-07 22:45 ` [PATCH v12 18/22] media/v4l2-core: pin_user_pages (FOLL_PIN) and put_user_page() conversion John Hubbard
2020-01-07 22:45 ` [PATCH v12 19/22] vfio, mm: " John Hubbard
2020-01-07 22:45 ` [PATCH v12 20/22] powerpc: book3s64: convert to pin_user_pages() and put_user_page() John Hubbard
2020-01-07 22:45 ` [PATCH v12 21/22] mm/gup_benchmark: use proper FOLL_WRITE flags instead of hard-coding "1" John Hubbard
2020-01-07 22:45 ` [PATCH v12 22/22] mm, tree-wide: rename put_user_page*() to unpin_user_page*() John Hubbard
2020-01-15 15:26   ` Christoph Hellwig
2020-01-09 22:07 ` [PATCH v12 00/22] mm/gup: prereqs to track dma-pinned pages: FOLL_PIN John Hubbard
2020-01-14 20:15   ` John Hubbard
2020-01-14 23:29     ` Andrew Morton

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