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From: Matthew Wilcox <willy@infradead.org>
To: Muchun Song <songmuchun@bytedance.com>
Cc: Jonathan Corbet <corbet@lwn.net>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com,
	dave.hansen@linux.intel.com, luto@kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	viro@zeniv.linux.org.uk,
	Andrew Morton <akpm@linux-foundation.org>,
	paulmck@kernel.org, mchehab+huawei@kernel.org,
	pawan.kumar.gupta@linux.intel.com,
	Randy Dunlap <rdunlap@infradead.org>,
	oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de,
	almasrymina@google.com, David Rientjes <rientjes@google.com>,
	linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	Linux Memory Management List <linux-mm@kvack.org>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [External] Re: [RFC PATCH 00/24] mm/hugetlb: Free some vmemmap pages of hugetlb page
Date: Tue, 15 Sep 2020 19:15:30 +0100	[thread overview]
Message-ID: <20200915181530.GL5449@casper.infradead.org> (raw)
In-Reply-To: <CAMZfGtW3S8kGJwff6oH14QWPXKTuQEAGdYwcLRUZxuJ7q8s7sA@mail.gmail.com>

On Wed, Sep 16, 2020 at 02:03:15AM +0800, Muchun Song wrote:
> On Wed, Sep 16, 2020 at 1:39 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Wed, Sep 16, 2020 at 01:32:46AM +0800, Muchun Song wrote:
> > > On Tue, Sep 15, 2020 at 11:42 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > >
> > > > On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote:
> > > > > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > > > >
> > > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote:
> > > > > > > This patch series will free some vmemmap pages(struct page structures)
> > > > > > > associated with each hugetlbpage when preallocated to save memory.
> > > > > >
> > > > > > It would be lovely to be able to do this.  Unfortunately, it's completely
> > > > > > impossible right now.  Consider, for example, get_user_pages() called
> > > > > > on the fifth page of a hugetlb page.
> > > > >
> > > > > Can you elaborate on the problem? Thanks so much.
> > > >
> > > > OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page
> > > > on a 4kB base page system.  Today, that results in a bio_vec containing
> > > > {head+5, 0, 0x800}.  Then we call page_to_phys() on that (head+5) struct
> > > > page to get the physical address of the I/O, and we turn it into a struct
> > > > scatterlist, which similarly has a reference to the page (head+5).
> > >
> > > As I know, in this case, the get_user_pages() will get a reference
> > > to the head page (head+0) before returning such that the hugetlb
> > > page can not be freed. Although get_user_pages() returns the
> > > page (head+5) and the scatterlist has a reference to the page
> > > (head+5), this patch series can handle this situation. I can not
> > > figure out where the problem is. What I missed? Thanks.
> >
> > You freed pages 4-511 from the vmemmap so they could be used for
> > something else.  Page 5 isn't there any more.  So if you return head+5,
> > then when we complete the I/O, we'll look for the compound_head() of
> > head+5 and we won't find head.
> 
> We do not free pages 4-511 from the vmemmap. Actually, we only
> free pages 128-511 from the vmemmap.
> 
> The 512 struct pages occupy 8 pages of physical memory. We only
> free 6 physical page frames to the buddy. But we will create a new
> mapping just like below. The virtual address of the freed pages will
> remap to the second page frame. So the second page frame is
> reused.

Oh!  I get what you're doing now.

For the vmemmap case, you free the last N-2 physical pages but map the
second physical page multiple times.  So for the 512 pages case, we
see pages:

abcdefgh | ijklmnop | ijklmnop | ijklmnop | ijklmnop | ijklmnop | ijklmnop ...

Huh.  I think that might work, except for PageHWPoison.  I'll go back
to your patch series and look at that some more.


  reply	other threads:[~2020-09-15 18:17 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-15 12:59 Muchun Song
2020-09-15 12:59 ` [RFC PATCH 01/24] mm/memory_hotplug: Move bootmem info registration API to bootmem_info.c Muchun Song
2020-09-29 22:46   ` Mike Kravetz
2020-09-15 12:59 ` [RFC PATCH 02/24] mm/memory_hotplug: Move {get,put}_page_bootmem() " Muchun Song
2020-09-29 23:30   ` Mike Kravetz
2020-09-15 12:59 ` [RFC PATCH 03/24] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP Muchun Song
2020-09-16  2:13   ` Randy Dunlap
2020-09-29 23:41   ` Mike Kravetz
2020-09-30  2:56     ` [External] " Muchun Song
2020-09-15 12:59 ` [RFC PATCH 04/24] mm/hugetlb: Register bootmem info when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP Muchun Song
2020-09-15 12:59 ` [RFC PATCH 05/24] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate Muchun Song
2020-09-30 22:41   ` Mike Kravetz
2020-10-01  2:57     ` [External] " Muchun Song
2020-09-15 12:59 ` [RFC PATCH 06/24] mm/hugetlb: Introduce pgtable allocation/freeing helpers Muchun Song
2020-09-15 12:59 ` [RFC PATCH 07/24] mm/hugetlb: Add freeing unused vmemmap pages support for x86 Muchun Song
2020-09-15 12:59 ` [RFC PATCH 08/24] mm/bootmem_info: Introduce {free,prepare}_vmemmap_page() Muchun Song
2020-09-15 12:59 ` [RFC PATCH 09/24] x86/mm: Introduce VMEMMAP_SIZE/VMEMMAP_END macro Muchun Song
2020-09-15 12:59 ` [RFC PATCH 10/24] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page Muchun Song
2020-09-15 12:59 ` [RFC PATCH 11/24] mm/hugetlb: Add vmemmap_pmd_huge macro for x86 Muchun Song
2020-09-15 12:59 ` [RFC PATCH 12/24] mm/hugetlb: Defer freeing of hugetlb pages Muchun Song
2020-09-15 12:59 ` [RFC PATCH 13/24] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page Muchun Song
2020-09-15 12:59 ` [RFC PATCH 14/24] mm/hugetlb: Introduce remap_huge_page_pmd_vmemmap helper Muchun Song
2020-09-15 12:59 ` [RFC PATCH 15/24] mm/hugetlb: Use PG_slab to indicate split pmd Muchun Song
2020-09-15 12:59 ` [RFC PATCH 16/24] mm/hugetlb: Support freeing vmemmap pages of gigantic page Muchun Song
2020-09-15 12:59 ` [RFC PATCH 17/24] mm/hugetlb: Add a BUILD_BUG_ON to check if struct page size is a power of two Muchun Song
2020-09-15 12:59 ` [RFC PATCH 18/24] mm/hugetlb: Clear PageHWPoison on the non-error memory page Muchun Song
2020-09-15 12:59 ` [RFC PATCH 19/24] mm/hugetlb: Flush work when dissolving hugetlb page Muchun Song
2020-09-15 12:59 ` [RFC PATCH 20/24] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap Muchun Song
2020-09-16  2:10   ` Randy Dunlap
2020-09-16  2:50     ` [External] " Muchun Song
2020-09-15 12:59 ` [RFC PATCH 21/24] mm/hugetlb: Merge pte to huge pmd only for gigantic page Muchun Song
2020-09-20  9:59   ` Muchun Song
2020-09-15 12:59 ` [RFC PATCH 22/24] mm/hugetlb: Implement vmemmap_pmd_mkhuge macro Muchun Song
2020-09-15 12:59 ` [RFC PATCH 23/24] mm/hugetlb: Gather discrete indexes of tail page Muchun Song
2020-09-15 12:59 ` [RFC PATCH 24/24] mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct page Muchun Song
2020-09-15 14:32 ` [RFC PATCH 00/24] mm/hugetlb: Free some vmemmap pages of hugetlb page Matthew Wilcox
2020-09-15 14:53   ` Dave Hansen
2020-09-15 15:28   ` [External] " Muchun Song
2020-09-15 15:42     ` Matthew Wilcox
2020-09-15 17:32       ` Muchun Song
2020-09-15 17:39         ` Matthew Wilcox
2020-09-15 18:03           ` Muchun Song
2020-09-15 18:15             ` Matthew Wilcox [this message]
2020-09-16  2:45               ` Muchun Song
2020-09-29 21:58 ` Mike Kravetz
2020-09-30  3:13   ` Matthew Wilcox
2020-10-07 21:12   ` Mike Kravetz
2020-10-09  4:13     ` [External] " Muchun Song

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