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From: Joao Martins <joao.m.martins@oracle.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: linux-mm@kvack.org, Dan Williams <dan.j.williams@intel.com>,
	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: Mon, 11 Oct 2021 16:53:29 +0100	[thread overview]
Message-ID: <01bf81a4-04f0-2ca3-0391-fceb1e557174@oracle.com> (raw)
In-Reply-To: <20211008115448.GA2976530@nvidia.com>

On 10/8/21 12:54, Jason Gunthorpe wrote:
> On Fri, Aug 27, 2021 at 03:58:13PM +0100, Joao Martins wrote:
>> @@ -2252,16 +2265,25 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
>>  			ret = 0;
>>  			break;
>>  		}
>> -		SetPageReferenced(page);
>> -		pages[*nr] = page;
>> -		if (unlikely(!try_grab_page(page, flags))) {
>> -			undo_dev_pagemap(nr, nr_start, flags, pages);
>> +
>> +		head = compound_head(page);
>> +		/* @end is assumed to be limited at most one compound page */
>> +		if (PageHead(head))
>> +			next = end;
>> +		refs = record_subpages(page, addr, next, pages + *nr);
>> +
>> +		SetPageReferenced(head);
>> +		if (unlikely(!try_grab_compound_head(head, refs, flags))) {
> 
> I was thinking about this some more, and this ordering doesn't seem
> like a good idea. We shouldn't be looking at any part of the struct
> page without holding the refcount, certainly not the compound_head()
> 
> The only optimization that might work here is to grab the head, then
> compute the extent of tail pages and amalgamate them. Holding a ref on
> the head also secures the tails.
> 

How about pmd_page(orig) / pud_page(orig) like what the rest of hugetlb/thp
checks do? i.e. we would pass pmd_page(orig)/pud_page(orig) to __gup_device_huge()
as an added @head argument. While keeping the same structure of counting tail pages
between @addr .. @end if we have a head page.

Albeit this lingers on whether it's OK to call PageHead() .. The PageHead policy is for
any page (PF_ANY) so no hidden calls to compound_head() when testing that page flag. but
in the end it accesses struct page flags which is well, still struct page data.

We could also check pgmap for a non-zero geometry (which tells us that pmd_page(orig) does
represent a head page). And that would save us from looking at struct page data today, but
it would introduce problems later whenever we remove the pgmap ref grab in gup_device_huge().

So the only viable might be to do the grab, count tails and fixup-ref like you suggest
above, and take the perf hit of one extra atomic op :(

It's interesting how THP (in gup_huge_pmd()) unilaterally computes tails assuming
pmd_page(orig) is the head page.

  reply	other threads:[~2021-10-11 15:54 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
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 [this message]
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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