Message-ID: <CAMZfGtVxiqwoxpYapVOK7GeyjZLzFY8EtD3h+4f1E-b6NwnNfA@mail.gmail.com>
Date:   Thu, 11 Mar 2021 12:13:58 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Chen Huang <chenhuang5@...wei.com>,
        Bodeddula Balasubramaniam <bodeddub@...zon.com>
Subject: Re: [External] Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap
 pages associated with each HugeTLB page

On Wed, Mar 10, 2021 at 10:21 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> On Mon, Mar 08, 2021 at 06:28:02PM +0800, Muchun Song wrote:
> > When we free a HugeTLB page to the buddy allocator, we need to allocate
> > the vmemmap pages associated with it. However, we may not be able to
> > allocate the vmemmap pages when the system is under memory pressure. In
> > this case, we just refuse to free the HugeTLB page. This changes behavior
> > in some corner cases as listed below:
> >
> >  1) Failing to free a huge page triggered by the user (decrease nr_pages).
> >
> >     User needs to try again later.
> >
> >  2) Failing to free a surplus huge page when freed by the application.
> >
> >     Try again later when freeing a huge page next time.
> >
> >  3) Failing to dissolve a free huge page on ZONE_MOVABLE via
> >     offline_pages().
> >
> >     This can happen when we have plenty of ZONE_MOVABLE memory, but
> >     not enough kernel memory to allocate vmemmmap pages.  We may even
> >     be able to migrate huge page contents, but will not be able to
> >     dissolve the source huge page.  This will prevent an offline
> >     operation and is unfortunate as memory offlining is expected to
> >     succeed on movable zones.  Users that depend on memory hotplug
> >     to succeed for movable zones should carefully consider whether the
> >     memory savings gained from this feature are worth the risk of
> >     possibly not being able to offline memory in certain situations.
>
> This is nice to have it here, but a normal user won't dig in the kernel to
> figure this out, so my question is: Do we have this documented somewhere under
> Documentation/?
> If not, could we document it there? It is nice to warn about this things were
> sysadmins can find them.

Make sense. I will do this.

>
> >  4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
> >     alloc_contig_range() - once we have that handling in place. Mainly
> >     affects CMA and virtio-mem.
> >
> >     Similar to 3). virito-mem will handle migration errors gracefully.
> >     CMA might be able to fallback on other free areas within the CMA
> >     region.
> >
> > Vmemmap pages are allocated from the page freeing context. In order for
> > those allocations to be not disruptive (e.g. trigger oom killer)
> > __GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
> > because a non sleeping allocation would be too fragile and it could fail
> > too easily under memory pressure. GFP_ATOMIC or other modes to access
> > memory reserves is not used because we want to prevent consuming
> > reserves under heavy hugetlb freeing.
> >
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > Tested-by: Chen Huang <chenhuang5@...wei.com>
> > Tested-by: Bodeddula Balasubramaniam <bodeddub@...zon.com>
>
> Sorry for jumping in late.
> It looks good to me:
>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>

Thanks.

>
> Minor request above and below:
>
> > ---
> >  Documentation/admin-guide/mm/hugetlbpage.rst |  8 +++
> >  include/linux/mm.h                           |  2 +
> >  mm/hugetlb.c                                 | 92 +++++++++++++++++++++-------
> >  mm/hugetlb_vmemmap.c                         | 32 ++++++----
> >  mm/hugetlb_vmemmap.h                         | 23 +++++++
> >  mm/sparse-vmemmap.c                          | 75 ++++++++++++++++++++++-
> >  6 files changed, 197 insertions(+), 35 deletions(-)
>
> [...]
>
>
>
> Could we place a brief comment about what we expect to return here?

OK. Will do.

>
> > -static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> >  {
> > -     return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> > +     unsigned long vmemmap_addr = (unsigned long)head;
> > +     unsigned long vmemmap_end, vmemmap_reuse;
> > +
> > +     if (!free_vmemmap_pages_per_hpage(h))
> > +             return 0;
> > +
> > +     vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > +     vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > +     vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > +     /*
> > +      * The pages which the vmemmap virtual address range [@vmemmap_addr,
> > +      * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> > +      * the range is mapped to the page which @vmemmap_reuse is mapped to.
> > +      * When a HugeTLB page is freed to the buddy allocator, previously
> > +      * discarded vmemmap pages must be allocated and remapping.
> > +      */
> > +     return vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> > +                                GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
> >  }
>
> --
> Oscar Salvador
> SUSE L3

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