Re: [PATCH v11 23/25] mm/gup: track FOLL_PIN pages

From: Jan Kara <jack@suse.cz>
To: John Hubbard <jhubbard@nvidia.com>
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>,
	"Christoph Hellwig" <hch@infradead.org>,
	"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>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Magnus Karlsson" <magnus.karlsson@intel.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Michal Hocko" <mhocko@suse.com>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	"Paul Mackerras" <paulus@samba.org>,
	"Shuah Khan" <shuah@kernel.org>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	bpf@vger.kernel.org, dri-devel@lists.freedesktop.org,
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	LKML <linux-kernel@vger.kernel.org>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v11 23/25] mm/gup: track FOLL_PIN pages
Date: Mon, 16 Dec 2019 13:53:53 +0100	[thread overview]
Message-ID: <20191216125353.GF22157@quack2.suse.cz> (raw)
In-Reply-To: <20191214032617.1670759-1-jhubbard@nvidia.com>

On Fri 13-12-19 19:26:17, John Hubbard wrote:
> Add tracking of pages that were pinned via FOLL_PIN.
> 
> As mentioned in the FOLL_PIN documentation, callers who effectively set
> FOLL_PIN are required to ultimately free such pages via unpin_user_page().
> The effect is similar to FOLL_GET, and may be thought of as "FOLL_GET
> for DIO and/or RDMA use".
> 
> Pages that have been pinned via FOLL_PIN are identifiable via a
> new function call:
> 
>    bool page_dma_pinned(struct page *page);
> 
> What to do in response to encountering such a page, is left to later
> patchsets. There is discussion about this in [1], [2], and [3].
> 
> This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask().
> 
> [1] Some slow progress on get_user_pages() (Apr 2, 2019):
>     https://lwn.net/Articles/784574/
> [2] DMA and get_user_pages() (LPC: Dec 12, 2018):
>     https://lwn.net/Articles/774411/
> [3] The trouble with get_user_pages() (Apr 30, 2018):
>     https://lwn.net/Articles/753027/
> 
> Suggested-by: Jan Kara <jack@suse.cz>
> Suggested-by: Jérôme Glisse <jglisse@redhat.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
> 
> Hi Jan,
> 
> This should address all of your comments for patch 23!

Thanks. One comment below:

> @@ -1486,6 +1500,10 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>  	VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
>  	if (flags & FOLL_TOUCH)
>  		touch_pmd(vma, addr, pmd, flags);
> +
> +	if (!try_grab_page(page, flags))
> +		return ERR_PTR(-ENOMEM);
> +
>  	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
>  		/*
>  		 * We don't mlock() pte-mapped THPs. This way we can avoid

I'd move this still a bit higher - just after VM_BUG_ON_PAGE() and before
if (flags & FOLL_TOUCH) test. Because touch_pmd() can update page tables
and we don't won't that if we're going to fail the fault.

With this fixed, the patch looks good to me so you can then add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR