From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Hugh Dickins <hughd@google.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Dave Hansen <dave.hansen@intel.com>,
Vlastimil Babka <vbabka@suse.cz>,
Christoph Lameter <cl@gentwo.org>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Jerome Marchand <jmarchan@redhat.com>,
Yang Shi <yang.shi@linaro.org>,
Sasha Levin <sasha.levin@oracle.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-fsdevel@vger.kernel.org
Subject: Re: [PATCHv4 04/25] rmap: support file thp
Date: Sat, 19 Mar 2016 04:01:01 +0300 [thread overview]
Message-ID: <20160319010101.GA29883@node.shutemov.name> (raw)
In-Reply-To: <87d1qs9lah.fsf@linux.vnet.ibm.com>
On Fri, Mar 18, 2016 at 03:10:06PM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> writes:
>
> > [ text/plain ]
> > Naive approach: on mapping/unmapping the page as compound we update
> > ->_mapcount on each 4k page. That's not efficient, but it's not obvious
> > how we can optimize this. We can look into optimization later.
> >
> > PG_double_map optimization doesn't work for file pages since lifecycle
> > of file pages is different comparing to anon pages: file page can be
> > mapped again at any time.
> >
>
> Can you explain this more ?. We added PG_double_map so that we can keep
> page_remove_rmap simpler. So if it isn't a compound page we still can do
>
> if (!atomic_add_negative(-1, &page->_mapcount))
>
> I am trying to understand why we can't use that with file pages ?
The first thing: for non-compound pages we still have simple
atomic_inc_and_test() / atomic_add_negative(-1), nothing changed here.
About compound pages:
For anon-THP PG_double_map allowed to not touch _mapcount in all subpages
until a PMD which maps the page is split. This way we significantly lower
overhead on refcounting as long as we have the page mapped with PMD-only,
since we only need to increment compound_mapcount().
The optimization is possible due to relatively simple lifecycle of
anonymous THP page:
- anon-THPs always mapped with PMD first;
- new mapping of THP can only be created via fork();
- the page only can get mapped with PTEs via split_huge_pmd();
For file-THP the situation is different. Once we allocated a huge page and
put it on radix tree, the page can be mapped with PTEs or PMDs at any
time. It makes the same optimization inapplicable there.
I think there *can* be some room for optimization, but I don't want to
invest more time here, until it's identified as bottleneck. It can lead to
more complex code on rmap side.
--
Kirill A. Shutemov
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next prev parent reply other threads:[~2016-03-19 1:01 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-11 22:58 [PATCHv4 00/25] THP-enabled tmpfs/shmem Kirill A. Shutemov
2016-03-11 22:58 ` [PATCHv4 01/25] mm: do not pass mm_struct into handle_mm_fault Kirill A. Shutemov
2016-03-11 22:58 ` [PATCHv4 02/25] mm: introduce fault_env Kirill A. Shutemov
2016-03-11 22:58 ` [PATCHv4 03/25] mm: postpone page table allocation until we have page to map Kirill A. Shutemov
2016-03-11 22:58 ` [PATCHv4 04/25] rmap: support file thp Kirill A. Shutemov
2016-03-18 9:40 ` Aneesh Kumar K.V
2016-03-19 1:01 ` Kirill A. Shutemov [this message]
2016-03-11 22:58 ` [PATCHv4 05/25] mm: introduce do_set_pmd() Kirill A. Shutemov
2016-03-11 22:58 ` [PATCHv4 06/25] mm, rmap: account file thp pages Kirill A. Shutemov
2016-03-15 15:30 ` [PATCHv5 " Kirill A. Shutemov
2016-03-11 22:58 ` [PATCHv4 07/25] thp, vmstats: add counters for huge file pages Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 08/25] thp: support file pages in zap_huge_pmd() Kirill A. Shutemov
2016-03-18 13:53 ` Aneesh Kumar K.V
2016-03-19 1:02 ` Kirill A. Shutemov
2016-03-21 4:33 ` Aneesh Kumar K.V
2016-03-21 14:39 ` Kirill A. Shutemov
2016-03-21 16:42 ` Aneesh Kumar K.V
2016-03-11 22:59 ` [PATCHv4 09/25] thp: handle file pages in split_huge_pmd() Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 10/25] thp: handle file COW faults Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 11/25] thp: handle file pages in mremap() Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 12/25] thp: skip file huge pmd on copy_huge_pmd() Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 13/25] thp: prepare change_huge_pmd() for file thp Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 14/25] thp: run vma_adjust_trans_huge() outside i_mmap_rwsem Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 15/25] thp: file pages support for split_huge_page() Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 16/25] thp, mlock: do not mlock PTE-mapped file huge pages Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 17/25] vmscan: split file huge pages before paging them out Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 18/25] page-flags: relax policy for PG_mappedtodisk and PG_reclaim Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 19/25] radix-tree: implement radix_tree_maybe_preload_order() Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 20/25] filemap: prepare find and delete operations for huge pages Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 21/25] truncate: handle file thp Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 22/25] shmem: prepare huge= mount option and sysfs knob Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 23/25] shmem: get_unmapped_area align huge page Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 24/25] shmem: add huge pages support Kirill A. Shutemov
2016-03-11 22:59 ` [PATCHv4 25/25] shmem, thp: respect MADV_{NO,}HUGEPAGE for file mappings Kirill A. Shutemov
2016-03-15 15:52 ` [PATCHv5 26/25] thp: update Documentation/vm/transhuge.txt Kirill A. Shutemov
2016-03-23 20:09 ` [PATCHv4 00/25] THP-enabled tmpfs/shmem Hugh Dickins
2016-03-24 9:17 ` Kirill A. Shutemov
2016-03-24 19:08 ` Hugh Dickins
2016-03-25 15:04 ` Kirill A. Shutemov
2016-03-26 0:00 ` Hugh Dickins
2016-03-28 18:00 ` Kirill A. Shutemov
2016-03-28 18:42 ` Hugh Dickins
2016-03-28 12:29 ` Kirill A. Shutemov
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