Re: [External] Re: [PATCH v15 4/8] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

On Mon 15-02-21 19:51:26, Muchun Song wrote:

On Mon, Feb 15, 2021 at 6:33 PM Michal Hocko [off-list ref] wrote:

quoted

On Mon 15-02-21 18:05:06, Muchun Song wrote:

quoted

On Fri, Feb 12, 2021 at 11:32 PM Michal Hocko [off-list ref] wrote:

[...]

quoted

quoted

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+int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+     int ret;
+     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.
+      */
+     ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
+                               GFP_ATOMIC | __GFP_NOWARN | __GFP_THISNODE);

I do not think that this is a good allocation mode. GFP_ATOMIC is a non
sleeping allocation and a medium memory pressure might cause it to
fail prematurely. I do not think this is really an atomic context which
couldn't afford memory reclaim. I also do not think we want to grant

Because alloc_huge_page_vmemmap is called under hugetlb_lock
now. So using GFP_ATOMIC indeed makes the code more simpler.

You can have a preallocated list of pages prior taking the lock.

A discussion about this can refer to here:

https://patchwork.kernel.org/project/linux-mm/patch/20210117151053.24600-5-songmuchun@bytedance.com/

I do not see any real response to the pre-allocation argument except
that put_page can be called from an atomic context. Which might be true
in general but it is not the case for hugetlb pages. hugetlb_lock would
have to be irq safe otherwise. Also the whole operation can be scheduled
to a kworker context for a stronger allocation context.

-- 
Michal Hocko
SUSE Labs