nvdimm.lists.linux.dev archive mirror
 help / color / mirror / Atom feed
From: Jason Gunthorpe <jgg@nvidia.com>
To: Joao Martins <joao.m.martins@oracle.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: Wed, 13 Oct 2021 14:41:40 -0300	[thread overview]
Message-ID: <20211013174140.GJ2744544@nvidia.com> (raw)
In-Reply-To: <01bf81a4-04f0-2ca3-0391-fceb1e557174@oracle.com>

On Mon, Oct 11, 2021 at 04:53:29PM +0100, Joao Martins wrote:
> On 10/8/21 12:54, Jason Gunthorpe wrote:

> > 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.

The right logic is what everything else does:

	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
	refs = record_subpages(page, addr, end, pages + *nr);
	head = try_grab_compound_head(pud_page(orig), refs, flags);

If you can use this, or not, depends entirely on answering the
question of why does  __gup_device_huge() exist at all.

This I don't fully know:

1) As discussed quite a few times now, the entire get_dev_pagemap
   stuff looks usless and should just be deleted. If you care about
   optimizing this I would persue doing that as it will give the
   biggest single win.

2) It breaks up the PUD/PMD into tail pages and scans them all
   Why? Can devmap combine multiple compound_head's into the same PTE?
   Does devmap guarentee that the PUD/PMD points to the head page? (I
   assume no)

But I'm looking at this some more and I see try_get_compound_head() is
reading the compound_head with no locking, just READ_ONCE, so that
must be OK under GUP.

It still seems to me the same generic algorithm should work
everywhere, once we get rid of the get_dev_pagemap

  start_pfn = pud/pmd_pfn() + pud/pmd_page_offset(addr)
  end_pfn = start_pfn + (end - addr) // fixme
  if (THP)
     refs = end_pfn - start_pfn
  if (devmap)
     refs = 1

  do {
     page = pfn_to_page(start_pfn)
     head_page = try_grab_compound_head(page, refs, flags)
     if (pud/pmd_val() != orig)
        err

     npages = 1 << compound_order(head_page)
     npages = min(npages, end_pfn - start_pfn)
     for (i = 0, iter = page; i != npages) {
     	 *pages++ = iter;
         mem_map_next(iter, page, i)
     }

     if (devmap && npages > 2)
         grab_compound_head(head_page, npages - 1, flags)
     start_pfn += npages;
  } while (start_pfn != end_pfn)

Above needs to be cleaned up quite a bit, but is the general idea.

There is an further optimization we can put in where we can know that
'page' is still in a currently grab'd compound (eg last_page+1 = page,
not past compound_order) and defer the refcount work.

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

I think this is an integral property of THP, probably not devmap/dax
though?

Jason

  reply	other threads:[~2021-10-13 17:41 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
2021-10-13 17:41       ` Jason Gunthorpe [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20211013174140.GJ2744544@nvidia.com \
    --to=jgg@nvidia.com \
    --cc=akpm@linux-foundation.org \
    --cc=corbet@lwn.net \
    --cc=dan.j.williams@intel.com \
    --cc=dave.jiang@intel.com \
    --cc=hch@lst.de \
    --cc=jane.chu@oracle.com \
    --cc=jhubbard@nvidia.com \
    --cc=joao.m.martins@oracle.com \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mike.kravetz@oracle.com \
    --cc=naoya.horiguchi@nec.com \
    --cc=nvdimm@lists.linux.dev \
    --cc=songmuchun@bytedance.com \
    --cc=vishal.l.verma@intel.com \
    --cc=willy@infradead.org \
    --subject='Re: [PATCH v4 08/14] mm/gup: grab head page refcount once for group of subpages' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).