linux-kernel - Re: Linux 4.9-rc6

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Date:   Sun, 20 Nov 2016 20:59:50 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 4.9-rc6

On Mon, 2016-11-21 at 01:35 +0000, Al Viro wrote:

> 
> Umm...  One possibility would be something like fs/namespace.c:m_start() -
> if nothing has changed since the last time, just use a cached pointer.
> That has sped the damn thing (/proc/mounts et.al.) big way, but it's
> dependent upon having an event count updated whenever we change the
> mount tree - doing the same for vma_area list might or might not be
> a good idea.  /proc/mounts and friends get ->poll() on that as well;
> that probably would _not_ be a good idea in this case.

Yes, a generation number could help in some cases.

Another potential issue with CONFIG_VMAP_STACK is that we make no
attempt to allocate 4 consecutive pages.

Even if we have plenty of memory, 4 calls to alloc_page() are likely to
give us 4 pages in completely different locations.

Here I printed the hugepage number of the 4 pages for some stacks :


0xffffc9001a07c000-0xffffc9001a081000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfcac Hfeba Hfec0 Hfc9d N0=4
0xffffc9001a084000-0xffffc9001a089000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfc79 Hfc79 Hfc79 Hfc83 N0=4
0xffffc9001a08c000-0xffffc9001a091000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfc9b Hfe91 Hfebe Hfca2 N0=4
0xffffc9001a094000-0xffffc9001a099000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfcaa Hfcaa Hfca6 Hfebc N0=4
0xffffc9001a09c000-0xffffc9001a0a1000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfe9b Hfe90 Hff09 Hfefb N0=4
0xffffc9001a0a4000-0xffffc9001a0a9000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfe94 Hfe62 Hfea0 Hfe7b N0=4
0xffffc9001a0ac000-0xffffc9001a0b1000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfe78 Hff05 Hff05 Hfc74 N0=4
0xffffc9001a0b4000-0xffffc9001a0b9000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfc9b Hfc9b Hfe83 Hf782 N0=4
0xffffc9001a0bc000-0xffffc9001a0c1000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfe78 Hfe78 Hfc7f Hfc7f N0=4
0xffffc9001a0c4000-0xffffc9001a0c9000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfebe Hfebe Hfe82 Hfe85 N0=4
0xffffc9001a0cc000-0xffffc9001a0d1000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfc6b Hfe62 Hfe62 Hfcaa N0=4
0xffffc9001a0d4000-0xffffc9001a0d9000   20480 _do_fork+0xe1/0x360 pages=4 vmalloc Hfebd Hfebd Hfc92 Hfc92 N0=4

This is a vmalloc() generic issue that is worth fixing now ?

Note this RFC might conflict with NUMA interleave policy.

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index f2481cb4e6b2..0123e97debb9 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1602,9 +1602,10 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 				 pgprot_t prot, int node)
 {
 	struct page **pages;
-	unsigned int nr_pages, array_size, i;
+	unsigned int nr_pages, array_size, i, j;
 	const gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO;
 	const gfp_t alloc_mask = gfp_mask | __GFP_NOWARN;
+	const gfp_t multi_alloc_mask = (gfp_mask & ~__GFP_DIRECT_RECLAIM) | __GFP_NORETRY;
 
 	nr_pages = get_vm_area_size(area) >> PAGE_SHIFT;
 	array_size = (nr_pages * sizeof(struct page *));
@@ -1624,20 +1625,34 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 		return NULL;
 	}
 
-	for (i = 0; i < area->nr_pages; i++) {
-		struct page *page;
-
-		if (node == NUMA_NO_NODE)
-			page = alloc_page(alloc_mask);
-		else
-			page = alloc_pages_node(node, alloc_mask, 0);
+	for (i = 0; i < area->nr_pages;) {
+		struct page *page = NULL;
+		unsigned int chunk_order = min(ilog2(area->nr_pages - i), MAX_ORDER - 1);
+
+		while (chunk_order && !page) {
+			if (node == NUMA_NO_NODE)
+				page = alloc_pages(multi_alloc_mask, chunk_order);
+			else
+				page = alloc_pages_node(node, multi_alloc_mask, chunk_order);
+			if (page)
+				split_page(page, chunk_order);
+			else
+				chunk_order--;
+		}
+		if (!page) {
+			if (node == NUMA_NO_NODE)
+				page = alloc_pages(alloc_mask, 0);
+			else
+				page = alloc_pages_node(node, alloc_mask, 0);
+		}
 
 		if (unlikely(!page)) {
 			/* Successfully allocated i pages, free them in __vunmap() */
 			area->nr_pages = i;
 			goto fail;
 		}
-		area->pages[i] = page;
+		for (j = 0; j < (1 << chunk_order); j++)
+			area->pages[i++] = page++;
 		if (gfpflags_allow_blocking(gfp_mask))
 			cond_resched();
 	}