On Mon, Feb 15, 2021 at 8:18 PM Michal Hocko [off-list ref] wrote:

On Mon 15-02-21 20:00:07, Muchun Song wrote:

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On Mon, Feb 15, 2021 at 7:51 PM Muchun Song [off-list ref] wrote:

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

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

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

[...]

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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/

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Moreover do we want to manipulate vmemmaps from under spinlock in
general. I have to say I have missed that detail when reviewing. Need to
think more.

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From the document of the kernel, I learned that __GFP_NOMEMALLOC
can be used to explicitly forbid access to emergency reserves. So if
we do not want to use the reserve memory. How about replacing it to

GFP_ATOMIC | __GFP_NOMEMALLOC | __GFP_NOWARN | __GFP_THISNODE

The whole point of GFP_ATOMIC is to grant access to memory reserves so
the above is quite dubious. If you do not want access to memory reserves

Look at the code of gfp_to_alloc_flags().

static inline unsigned int gfp_to_alloc_flags(gfp_t gfp_mask)
{
        [...]
        if (gfp_mask & __GFP_ATOMIC) {
        /*
         * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
         * if it can't schedule.
         */
        if (!(gfp_mask & __GFP_NOMEMALLOC))
                alloc_flags |= ALLOC_HARDER;
       [...]
}

Seems to allow this operation (GFP_ATOMIC | __GFP_NOMEMALLOC).

Please read my response again more carefully. I am not claiming that
combination is not allowed. I have said it doesn't make any sense in
this context.

I see you are worried that using GFP_ATOMIC will use reverse memory
unlimited. So I think that __GFP_NOMEMALLOC may be suitable for us.
Sorry, I may not understand the point you said. What I missed?

--
Michal Hocko
SUSE Labs