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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>