Thread (2 messages) 2 messages, 2 authors, 2012-02-28

Re: [PATCH] Correct alignment of huge page requests.

From: Andrew Morton <akpm@linux-foundation.org>
Date: 2012-02-28 20:26:19
Also in: lkml

On Mon, 27 Feb 2012 23:00:28 -0500
Steven Truelove [off-list ref] wrote:
When calling shmget() with SHM_HUGETLB, shmget aligns the request size to PAGE_SIZE, but this is not sufficient.  Modified hugetlb_file_setup() to align requests to the huge page size.  Also modified mmap_pgoff() to avoid duplicating this check and to align against the start address.
I don't think this is quite right.

Suppose huge_page_size is 4096, addr=4095, len=4098.  So we're mapping
three pages: the last byte of the first page, all of the second page
and the first byte of the third page. 
quoted hunk ↗ jump to hunk
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -938,6 +938,8 @@ struct file *hugetlb_file_setup(const char *name, size_t size,
 	struct path path;
 	struct dentry *root;
 	struct qstr quick_string;
+	struct hstate *hstate;
+	int num_pages;
 
 	*user = NULL;
 	if (!hugetlbfs_vfsmount)
@@ -967,10 +969,11 @@ struct file *hugetlb_file_setup(const char *name, size_t size,
 	if (!inode)
 		goto out_dentry;
 
+	hstate = hstate_inode(inode);
+	num_pages = ALIGN(size, huge_page_size(hstate)) >>
+			huge_page_shift(hstate);
 	error = -ENOMEM;
-	if (hugetlb_reserve_pages(inode, 0,
-			size >> huge_page_shift(hstate_inode(inode)), NULL,
-			acctflag))
+	if (hugetlb_reserve_pages(inode, 0, num_pages, NULL, acctflag))
 		goto out_inode;
 
 	d_instantiate(path.dentry, inode);
diff --git a/mm/mmap.c b/mm/mmap.c
index 3f758c7..1f44ccf 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1098,8 +1098,12 @@ SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
 		 * taken when vm_ops->mmap() is called
 		 * A dummy user value is used because we are not locking
 		 * memory so no accounting is necessary
+		 * Length is increased by the amount necessary to align
+		 * the base address to the huge page size.
+		 * hugetlb_file_setup() aligns the end of the buffer to
+		 * the huge page size.
 		 */
-		len = ALIGN(len, huge_page_size(&default_hstate));
+		len += ALIGN(addr, huge_page_size(&default_hstate)) - addr;
 		file = hugetlb_file_setup(HUGETLB_ANON_FILE, len, VM_NORESERVE,
 						&user, HUGETLB_ANONHUGE_INODE);
mmap_pgoff() will change `len' from 4098 to 4099.  hugetlb_file_setup()
will round that up to 8192 and will decide to reserve two pages, not
three.

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