Re: [PATCH] mm, hugetlb: Avoid double clearing for hugetlb pages

[Michal Hocko <mhocko@suse.com>]

On Mon 19-10-20 15:28:53, Guilherme G. Piccoli wrote:
[...]
> $ time echo 32768 > /proc/sys/vm/nr_hugepages
> real    0m24.189s
> user    0m0.000s
> sys     0m24.184s
> 
> $ cat /proc/meminfo |grep "MemA\|Hugetlb"
> MemAvailable:   30784732 kB
> Hugetlb:        67108864 kB
> 
> * Without this patch, init_on_alloc=0
> $ cat /proc/meminfo |grep "MemA\|Hugetlb"
> MemAvailable:   97892752 kB
> Hugetlb:               0 kB
> 
> $ time echo 32768 > /proc/sys/vm/nr_hugepages
> real    0m0.316s
> user    0m0.000s
> sys     0m0.316s

Yes zeroying is quite costly and that is to be expected when the feature
is enabled. Hugetlb like other allocator users perform their own
initialization rather than go through __GFP_ZERO path. More on that
below.

Could you be more specific about why this is a problem. Hugetlb pool is
usualy preallocatd once during early boot. 24s for 65GB of 2MB pages
is non trivial amount of time but it doens't look like a major disaster
either. If the pool is allocated later it can take much more time due to
memory fragmentation.

I definitely do not want to downplay this but I would like to hear about
the real life examples of the problem.

[...]
> 
> Hi everybody, thanks in advance for the review/comments. I'd like to
> point 2 things related to the implementation:
> 
> 1) I understand that adding GFP flags is not really welcome by the
> mm community; I've considered passing that as function parameter but
> that would be a hacky mess, so I decided to add the flag since it seems
> this is a fair use of the flag mechanism (to control actions on pages).
> If anybody has a better/simpler suggestion to implement this, I'm all
> ears - thanks!

This has been discussed already (http://lkml.kernel.org/r/20190514143537.10435-4-glider@google.com.
Previously it has been brought up in SLUB context AFAIR. Your numbers
are quite clear here but do we really need a gfp flag with all the
problems we tend to grow in with them?

One potential way around this specifically for hugetlb would be to use
__GFP_ZERO when allocating from the allocator and marking the fact in
the struct page while it is sitting in the pool. Page fault handler
could then skip the zeroying phase. Not an act of beauty TBH but it
fits into the existing model of the full control over initialization.
Btw. it would allow to implement init_on_free semantic as well. I
haven't implemented the actual two main methods
hugetlb_test_clear_pre_init_page and hugetlb_mark_pre_init_page because
I am not entirely sure about the current state of hugetlb struct page in
the pool. But there should be a lot of room in there (or in tail pages).
Mike will certainly know much better. But the skeleton of the patch
would look like something like this (not even compile tested).

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index b5c109703daa..031af7cdf8a7 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -724,7 +724,8 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
 			error = PTR_ERR(page);
 			goto out;
 		}
-		clear_huge_page(page, addr, pages_per_huge_page(h));
+		if (!hugetlb_test_clear_pre_init_page(page))
+			clear_huge_page(page, addr, pages_per_huge_page(h));
 		__SetPageUptodate(page);
 		error = huge_add_to_page_cache(page, mapping, index);
 		if (unlikely(error)) {
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 67fc6383995b..83cc8abb4d69 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1413,6 +1413,7 @@ static void __free_huge_page(struct page *page)
 	page->mapping = NULL;
 	restore_reserve = PagePrivate(page);
 	ClearPagePrivate(page);
+	hugetlb_test_clear_pre_init_page(page);
 
 	/*
 	 * If PagePrivate() was set on page, page allocation consumed a
@@ -1703,6 +1704,7 @@ static struct page *alloc_buddy_huge_page(struct hstate *h,
 	int order = huge_page_order(h);
 	struct page *page;
 	bool alloc_try_hard = true;
+	bool pre_init = false;
 
 	/*
 	 * By default we always try hard to allocate the page with
@@ -1718,10 +1720,18 @@ static struct page *alloc_buddy_huge_page(struct hstate *h,
 		gfp_mask |= __GFP_RETRY_MAYFAIL;
 	if (nid == NUMA_NO_NODE)
 		nid = numa_mem_id();
+
+	/* prevent from double initialization */
+	if (want_init_on_alloc(gfp_mask)) {
+		gfp_mask |= __GFP_ZERO;
+		pre_init = true;
+	}
+
 	page = __alloc_pages_nodemask(gfp_mask, order, nid, nmask);
-	if (page)
+	if (page) {
 		__count_vm_event(HTLB_BUDDY_PGALLOC);
-	else
+		hugetlb_mark_pre_init_page(page);
+	} else
 		__count_vm_event(HTLB_BUDDY_PGALLOC_FAIL);
 
 	/*
@@ -4221,6 +4231,7 @@ static vm_fault_t hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
 		goto out_release_all;
 	}
 
+	hugetlb_test_clear_pre_init_page(new_page);
 	copy_user_huge_page(new_page, old_page, address, vma,
 			    pages_per_huge_page(h));
 	__SetPageUptodate(new_page);
@@ -4411,7 +4422,8 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 			ret = vmf_error(PTR_ERR(page));
 			goto out;
 		}
-		clear_huge_page(page, address, pages_per_huge_page(h));
+		if (!hugetlb_test_clear_pre_init_page(page))
+			clear_huge_page(page, address, pages_per_huge_page(h));
 		__SetPageUptodate(page);
 		new_page = true;
 
@@ -4709,6 +4721,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 		if (IS_ERR(page))
 			goto out;
 
+		hugetlb_test_clear_pre_init_page(page);
 		ret = copy_huge_page_from_user(page,
 						(const void __user *) src_addr,
 						pages_per_huge_page(h), false);
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index eddbe4e56c73..8cc1fc9c4d13 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -525,7 +525,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	unsigned long flags = qp->flags;
 	int ret;
 	bool has_unmovable = false;
-	pte_t *pte;
+	pte_t *pte, *mapped_pte;
 	spinlock_t *ptl;
 
 	ptl = pmd_trans_huge_lock(pmd, vma);
@@ -539,7 +539,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	if (pmd_trans_unstable(pmd))
 		return 0;
 
-	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
+	mapped_pte = pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
 	for (; addr != end; pte++, addr += PAGE_SIZE) {
 		if (!pte_present(*pte))
 			continue;
@@ -571,7 +571,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 		} else
 			break;
 	}
-	pte_unmap_unlock(pte - 1, ptl);
+	pte_unmap_unlock(mapped_pte, ptl);
 	cond_resched();
 
 	if (has_unmovable)
-- 
Michal Hocko
SUSE Labs