Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Oscar Salvador]

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