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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: Alistair Popple <apopple@nvidia.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Felix Kuehling <Felix.Kuehling@amd.com>,
	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 18:55:24 -0300	[thread overview]
Message-ID: <20210930215524.GI3544071@ziepe.ca> (raw)
In-Reply-To: <2ab374d1-4cc1-60a4-6663-81de7d59667b@oracle.com>

On Thu, Sep 30, 2021 at 06:54:05PM +0100, Joao Martins wrote:
> On 9/30/21 04:01, Alistair Popple wrote:
> > 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.
>
> IIUC Dan's reasoning was that fsdax wasn't able to deal with
> surprise removal [1] so his patches were to ensure fsdax (or the
> pmem block device) poisons/kills the pages as a way to notify
> filesystem/dm that the page was to be kept unmapped:

Sure, but that has nothing to do with GUP, that is between the
filesytem and fsdax
 
> But if fsdax doesn't wait for all the pgmap references[*] on its
> pagemap cleanup callback then what's the pgmap ref in
> __gup_device_huge() pairs/protects us up against that is specific to
> fsdax?

It does wait for refs

It sets the pgmap.ref to:

	pmem->pgmap.ref = &q->q_usage_counter;

And that ref is incr'd by the struct page lifetime - the unincr is in
__put_page() above

fsdax_pagemap_ops does pmem_pagemap_kill() which calls
blk_freeze_queue_start() which does percpu_ref_kill(). Then the
pmem_pagemap_cleanup() eventually does blk_mq_freeze_queue_wait()
which will sleep until the prefcpu ref reaches zero.

In other words fsdax cannot pass cleanup while a struct page exists
with a non-zero refcount, which answers Alistair's question about how
fsdax's cleanup work.

Jason

  reply	other threads:[~2021-09-30 21:55 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
2021-09-30 17:54                       ` Joao Martins
2021-09-30 21:55                         ` Jason Gunthorpe [this message]
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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