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SubjectRe: [PATCH] HUGETLBFS: Align memory request to multiple of huge page size to avoid underallocating.
On Mon, 27 Feb 2012 09:09:28 -0500
Steven Truelove <steven.truelove@utoronto.ca> 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.
>
> Signed-off-by: Steven Truelove <steven.truelove@utoronto.ca>
> ---
> fs/hugetlbfs/inode.c | 9 ++++++---
> 1 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 1e85a7a..6c23f09 100644
> --- 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 = (size + huge_page_size(hstate) - 1) >>
> + 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);

A few things...

- sys_mmap_pgoff() does the rounding up prior to calling
hugetlb_file_setup(). ipc/shm.c:newseg() does not.

We should be consistent here: do it in the caller or the callee,
not both (or neither!). I guess doing it in the callee would be
best.

- The above code could/should have used ALIGN(). Or round_up(): the
difference presently escapes me, even though it was so obvious that
we left all these things undocumented.

- What's the point in aligning the length if we don't also look at
the start address? If that isn't a multiple of huge_page_size(), we
will need an additional page.



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