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.
> 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@xxxxxxxxxxxxx>
> Tested-by: Chen Huang <chenhuang5@xxxxxxxxxx>
> Tested-by: Bodeddula Balasubramaniam <bodeddub@xxxxxxxxxx>
Sorry for jumping in late.
It looks good to me:
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
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?
> -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
