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.

 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.

[mike.kravetz@oracle.com: fix dissolve_free_huge_page use of tail/head page]
  Link: https://lkml.kernel.org/r/20210527231225.226987-1-mike.kravetz@oracle.com
[willy@infradead.org: fix alloc_vmemmap_page_list documentation warning]
  Link: https://lkml.kernel.org/r/20210615200242.1716568-6-willy@infradead.org

Link: https://lkml.kernel.org/r/20210510030027.56044-7-songmuchun@bytedance.comSigned-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Barry Song <song.bao.hua@hisilicon.com>
Cc: Bodeddula Balasubramaniam <bodeddub@amazon.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chen Huang <chenhuang5@huawei.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: HORIGUCHI NAOYA <naoya.horiguchi@nec.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Oliver Neukum <oneukum@suse.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>