Date:   Thu, 17 May 2018 13:53:01 +0800
From:   Chao Fan <fanc.fnst@...fujitsu.com>
To:     Baoquan He <bhe@...hat.com>
CC:     <linux-kernel@...r.kernel.org>, <mingo@...nel.org>,
        <lcapitulino@...hat.com>, <keescook@...omium.org>,
        <tglx@...utronix.de>, <x86@...nel.org>, <hpa@...or.com>,
        <yasu.isimatu@...il.com>, <indou.takao@...fujitsu.com>,
        <douly.fnst@...fujitsu.com>
Subject: Re: [PATCH 1/2] x86/boot/KASLR: Add two functions for 1GB huge pages
 handling

On Thu, May 17, 2018 at 12:03:43PM +0800, Baoquan He wrote:
>Hi Chao,
>
>On 05/17/18 at 11:27am, Chao Fan wrote:
>> >+/* Store the number of 1GB huge pages which user specified.*/
>> >+static unsigned long max_gb_huge_pages;
>> >+
>> >+static int parse_gb_huge_pages(char *param, char* val)
>> >+{
>> >+	char *p;
>> >+	u64 mem_size;
>> >+	static bool gbpage_sz = false;
>> >+
>> >+	if (!strcmp(param, "hugepagesz")) {
>> >+		p = val;
>> >+		mem_size = memparse(p, &p);
>> >+		if (mem_size == PUD_SIZE) {
>> >+			if (gbpage_sz)
>> >+				warn("Repeadly set hugeTLB page size of 1G!\n");
>> >+			gbpage_sz = true;
>> >+		} else
>> >+			gbpage_sz = false;
>> >+	} else if (!strcmp(param, "hugepages") && gbpage_sz) {
>> >+		p = val;
>> >+		max_gb_huge_pages = simple_strtoull(p, &p, 0);
>> >+		debug_putaddr(max_gb_huge_pages);
>> >+	}
>> >+}
>> >+
>> >+
>> > static int handle_mem_memmap(void)
>> > {
>> > 	char *args = (char *)get_cmd_line_ptr();
>> >@@ -466,6 +492,51 @@ static void store_slot_info(struct mem_vector *region, unsigned long image_size)
>> > 	}
>> > }
>> > 
>> >+/* Skip as many 1GB huge pages as possible in the passed region. */
>> >+static void process_gb_huge_page(struct mem_vector *region, unsigned long image_size)
>> >+{
>> >+	int i = 0;
>> >+	unsigned long addr, size;
>> >+	struct mem_vector tmp;
>> >+
>> >+	if (!max_gb_huge_pages) {
>> >+		store_slot_info(region, image_size);
>> >+		return;
>> >+	}
>> >+
>> >+	addr = ALIGN(region->start, PUD_SIZE);
>> >+	/* If Did we raise the address above the passed in memory entry? */
>> >+	if (addr < region->start + region->size)
>> >+		size = region->size - (addr - region->start);
>> >+
>> >+	/* Check how many 1GB huge pages can be filtered out*/
>> >+	while (size > PUD_SIZE && max_gb_huge_pages) {
>> >+		size -= PUD_SIZE;
>> >+		max_gb_huge_pages--;
>> 
>> The global variable 'max_gb_huge_pages' means how many huge pages
>> user specified when you get it from command line.
>> But here, everytime we find a position which is good for huge page
>> allocation, the 'max_gdb_huge_page' decreased. So in my understanding,
>> it is used to store how many huge pages that we still need to search memory
>> for good slots to filter out, right?
>> If it's right, maybe the name 'max_gb_huge_pages' is not very suitable.
>> If my understanding is wrong, please tell me.
>
>No, you have understood it very right. I finished the draft patch last
>week, but changed this variable name and the function names several
>time, still I feel they are not good. However I can't get a better name.
>
>Yes, 'max_gb_huge_pages' stores how many 1GB huge pages are expected
>from kernel command-line. And in this function it will be decreased. But
>we can't define another global variable only for decreasing in this
>place.
>
>And you can see that in this patchset I only take cares of 1GB huge
>pages. While on x86 we have two kinds of huge pages, 2MB and 1GB, why
>1GB only? Because 2MB is not impacted by KASLR, please check the code in 
>hugetlb_nrpages_setup() of mm/hugetlb.c . Only 1GB huge pages need be
>pre-allocated in hugetlb_nrpages_setup(), and if you look into
>hugetlb_nrpages_setup(), you will find that it will call
>alloc_bootmem_huge_page() to allocate huge pages one by one, but not at
>one time. That is why I always add 'gb' in the middle of the global
>variable and the newly added functions.
>
>And it will answer your below questions. When walk over all memory
>regions, 'max_gb_huge_pages' is still not 0, what should we do? It's
>normal and done as expected. Here hugetlb only try its best to allocate
>as many as possible according to 'max_gb_huge_pages'. If can't fully
>satisfied, it's fine. E.g on bare-metal machine with 16GB RAM, you add
>below to command-line:
>
>default_hugepagesz=1G hugepagesz=1G hugepages=20
>
>Then it will get 14 good 1GB huge pages with kaslr disabled since [0,1G)
>and [3G,4G) are touched by bios reservation and pci/firmware reservation.
>Then this 14 huge pages are maximal value which is expected. It's not a
>bug in huge page. But with kaslr enabled, it sometime only get 13 1GB
>huge pages because KASLR put kernel into one of those good 1GB huge
>pages. This is a bug.

Thanks for your explaination, I got it.

>
>I am not very familiar with huge page handling, just read code recently
>because of this kaslr bug. Hope Luiz and people from his team can help
>correct and clarify if anything is not right. Especially the function
>names, I feel it's not good, if anyone have a better idea, I will really
>appreciate that.
>> 
>> >+		i++;
>> >+	}
>> >+
>> >+	if (!i) {
>> >+		store_slot_info(region, image_size);
>> >+		return;
>> >+	}
>> >+
>> >+	/* Process the remaining regions after filtering out. */
>> >+
>> This line may be unusable.
>
>Hmm, I made it on purpose. Because 1GB huge pages may be digged out from
>the middle, then the remaing head and tail regions still need be
>handled. I put it here to mean that it covers below two code blocks. 
>

Yes, the two parts below are all in the condition when if(!i) is false.
The first part is the memory before good slots for huge pages,
the second part is after.

>I can remove it if people think it's not appropriate.
>
>> >+	if (addr >= region->start + image_size) {
>> >+		tmp.start = region->start;
>> >+		tmp.size = addr - region->start;
>> >+		store_slot_info(&tmp, image_size);
>> >+	}
>> >+
>> >+	size  = region->size - (addr - region->start) - i * PUD_SIZE;
>> >+        if (size >= image_size) {
>> >+		tmp.start = addr + i*PUD_SIZE;
>> >+		tmp.size = size;
>> >+		store_slot_info(&tmp, image_size);
>> >+        }
These 5 lines may have a wrong space, you can check it.

Thanks,
Chao Fan

>> 
>> I have another question not related to kaslr.
>> Here you try to avoid the memory from addr to (addr + i * PUD_SIZE),
>> but I wonder if after walking all memory regions, 'max_gb_huge_pages'
>> is still more than 0, which means there isn't enough memory slots for
>> huge page, what will happen?
>
>Please check the response at the beginning of response.
>
>Thanks
>Baoquan
>
>> 
>> 
>> >+}
>> >+
>> > static unsigned long slots_fetch_random(void)
>> > {
>> > 	unsigned long slot;
>> >-- 
>> >2.13.6
>> >
>> >
>> >
>> 
>> 
>
>

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