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From: Alistair Popple <apopple@nvidia.com>
To: Joao Martins <joao.m.martins@oracle.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Felix Kuehling <Felix.Kuehling@amd.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>
Cc: <linux-mm@kvack.org>, Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	Naoya Horiguchi <naoya.horiguchi@nec.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Jane Chu <jane.chu@oracle.com>,
	Muchun Song <songmuchun@bytedance.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jonathan Corbet <corbet@lwn.net>, Christoph Hellwig <hch@lst.de>,
	<nvdimm@lists.linux.dev>, <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v4 08/14] mm/gup: grab head page refcount once for group of subpages
Date: Thu, 30 Sep 2021 13:01:14 +1000	[thread overview]
Message-ID: <31536278.clc0Zd3cv0@nvdebian> (raw)
In-Reply-To: <20210929193405.GZ3544071@ziepe.ca>

On Thursday, 30 September 2021 5:34:05 AM AEST Jason Gunthorpe wrote:
> On Wed, Sep 29, 2021 at 12:50:15PM +0100, Joao Martins wrote:
> 
> > > If the get_dev_pagemap has to remain then it just means we have to
> > > flush before changing pagemap pointers
> > Right -- I don't think we should need it as that discussion on the other
> > thread goes.
> > 
> > OTOH, using @pgmap might be useful to unblock gup-fast FOLL_LONGTERM
> > for certain devmap types[0] (like MEMORY_DEVICE_GENERIC [device-dax]
> > can support it but not MEMORY_DEVICE_FSDAX [fsdax]).
> 
> When looking at Logan's patches I think it is pretty clear to me that
> page->pgmap must never be a dangling pointer if the caller has a
> legitimate refcount on the page.
> 
> For instance the migrate and stuff all blindly calls
> is_device_private_page() on the struct page expecting a valid
> page->pgmap.
> 
> This also looks like it is happening, ie
> 
> void __put_page(struct page *page)
> {
> 	if (is_zone_device_page(page)) {
> 		put_dev_pagemap(page->pgmap);
> 
> Is indeed putting the pgmap ref back when the page becomes ungettable.
> 
> This properly happens when the page refcount goes to zero and so it
> should fully interlock with __page_cache_add_speculative():
> 
> 	if (unlikely(!page_ref_add_unless(page, count, 0))) {
> 
> Thus, in gup.c, if we succeed at try_grab_compound_head() then
> page->pgmap is a stable pointer with a valid refcount.
> 
> So, all the external pgmap stuff in gup.c is completely pointless.
> try_grab_compound_head() provides us with an equivalent protection at
> lower cost. Remember gup.c doesn't deref the pgmap at all.
> 
> Dan/Alistair/Felix do you see any hole in that argument??

As background note that device pages are currently considered free when
refcount == 1 but the pgmap reference is dropped when the refcount transitions
1->0. The final pgmap reference is typically dropped when a driver calls
memunmap_pages() and put_page() drops the last page reference:

void memunmap_pages(struct dev_pagemap *pgmap)
{
        unsigned long pfn;
        int i;

        dev_pagemap_kill(pgmap);
        for (i = 0; i < pgmap->nr_range; i++)
                for_each_device_pfn(pfn, pgmap, i)
                        put_page(pfn_to_page(pfn));
        dev_pagemap_cleanup(pgmap);

If there are still pgmap references dev_pagemap_cleanup(pgmap) will block until
the final reference is dropped. So I think your argument holds at least for
DEVICE_PRIVATE and DEVICE_GENERIC. DEVICE_FS_DAX defines it's own pagemap
cleanup but I can't see why the same argument wouldn't hold there - if a page
has a valid refcount it must have a reference on the pagemap too.

 - Alistair

> So lets just delete it!
> 
> Jason
> 





  reply	other threads:[~2021-09-30  3:01 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-27 14:58 [PATCH v4 00/14] mm, sparse-vmemmap: Introduce compound devmaps for device-dax Joao Martins
2021-08-27 14:58 ` [PATCH v4 01/14] memory-failure: fetch compound_head after pgmap_pfn_valid() Joao Martins
2021-08-27 14:58 ` [PATCH v4 02/14] mm/page_alloc: split prep_compound_page into head and tail subparts Joao Martins
2021-08-27 14:58 ` [PATCH v4 03/14] mm/page_alloc: refactor memmap_init_zone_device() page init Joao Martins
2021-08-27 14:58 ` [PATCH v4 04/14] mm/memremap: add ZONE_DEVICE support for compound pages Joao Martins
2021-08-27 15:33   ` Christoph Hellwig
2021-08-27 16:00     ` Joao Martins
2021-09-01  9:44       ` Christoph Hellwig
2021-09-09  9:38         ` Joao Martins
2021-08-27 14:58 ` [PATCH v4 05/14] device-dax: use ALIGN() for determining pgoff Joao Martins
2021-08-27 14:58 ` [PATCH v4 06/14] device-dax: ensure dev_dax->pgmap is valid for dynamic devices Joao Martins
2021-11-05  0:31   ` Dan Williams
2021-11-05 12:09     ` Joao Martins
2021-11-05 16:14       ` Joao Martins
2021-11-05 16:46       ` Dan Williams
2021-11-05 18:11         ` Joao Martins
2021-08-27 14:58 ` [PATCH v4 07/14] device-dax: compound devmap support Joao Martins
2021-11-05  0:38   ` Dan Williams
2021-11-05 14:10     ` Joao Martins
2021-11-05 16:41       ` Dan Williams
2021-08-27 14:58 ` [PATCH v4 08/14] mm/gup: grab head page refcount once for group of subpages Joao Martins
2021-08-27 16:25   ` Jason Gunthorpe
2021-08-27 18:34     ` Joao Martins
2021-08-30 13:07       ` Jason Gunthorpe
2021-08-31 12:34         ` Joao Martins
2021-08-31 17:05           ` Jason Gunthorpe
2021-09-23 16:51             ` Joao Martins
2021-09-28 18:01               ` Jason Gunthorpe
2021-09-29 11:50                 ` Joao Martins
2021-09-29 19:34                   ` Jason Gunthorpe
2021-09-30  3:01                     ` Alistair Popple [this message]
2021-09-30 17:54                       ` Joao Martins
2021-09-30 21:55                         ` Jason Gunthorpe
2021-10-18 18:36                       ` Jason Gunthorpe
2021-10-18 18:37                   ` Jason Gunthorpe
2021-10-08 11:54   ` Jason Gunthorpe
2021-10-11 15:53     ` Joao Martins
2021-10-13 17:41       ` Jason Gunthorpe
2021-10-13 19:18         ` Joao Martins
2021-10-13 19:43           ` Jason Gunthorpe
2021-10-14 17:56             ` Joao Martins
2021-10-14 18:06               ` Jason Gunthorpe
2021-08-27 14:58 ` [PATCH v4 09/14] mm/sparse-vmemmap: add a pgmap argument to section activation Joao Martins
2021-08-27 14:58 ` [PATCH v4 10/14] mm/sparse-vmemmap: refactor core of vmemmap_populate_basepages() to helper Joao Martins
2021-08-27 14:58 ` [PATCH v4 11/14] mm/hugetlb_vmemmap: move comment block to Documentation/vm Joao Martins
2021-08-27 14:58 ` [PATCH v4 12/14] mm/sparse-vmemmap: populate compound devmaps Joao Martins
2021-08-27 14:58 ` [PATCH v4 13/14] mm/page_alloc: reuse tail struct pages for " Joao Martins
2021-08-27 14:58 ` [PATCH v4 14/14] mm/sparse-vmemmap: improve memory savings for compound pud geometry Joao Martins

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