Re: [PATCH] hugetlbfs read() support

Badari Pulavarty
2007-07-19T14:44:13Z

On Wed, 2007-07-18 at 22:19 -0700, Andrew Morton wrote:
> On Fri, 13 Jul 2007 18:23:33 -0700 Badari Pulavarty <pbadari@us.ibm.com> wrote:
>
> > Hi Andrew,
> >
> > Here is the patch to support read() for hugetlbfs, needed to get
> > oprofile working on executables backed by largepages.
> >
> > If you plan to consider Christoph Lameter's pagecache cleanup patches,
> > I will re-write this. Otherwise, please consider this for -mm.
> >
> > Thanks,
> > Badari
> >
> > Support for reading from hugetlbfs files. libhugetlbfs lets application
> > text/data to be placed in large pages. When we do that, oprofile doesn't
> > work - since libbfd tries to read from it.
> >
> > This code is very similar to what do_generic_mapping_read() does, but
> > I can't use it since it has PAGE_CACHE_SIZE assumptions.
> >
> > Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
> > Acked-by: William Irwin <bill.irwin@oracle.com>
> > Tested-by: Nishanth Aravamudan <nacc@us.ibm.com>
> >
> > fs/hugetlbfs/inode.c | 113 +++++++++++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 113 insertions(+)
> >
> > Index: linux-2.6.22/fs/hugetlbfs/inode.c
> > ===================================================================
> > --- linux-2.6.22.orig/fs/hugetlbfs/inode.c 2007-07-08 16:32:17.000000000 -0700
> > +++ linux-2.6.22/fs/hugetlbfs/inode.c 2007-07-13 19:24:36.000000000 -0700
> > @@ -156,6 +156,118 @@ full_search:
> > }
> > #endif
> >
> > +static int
> > +hugetlbfs_read_actor(struct page *page, unsigned long offset,
> > + char __user *buf, unsigned long count,
> > + unsigned long size)
> > +{
> > + char *kaddr;
> > + unsigned long left, copied = 0;
> > + int i, chunksize;
> > +
> > + if (size > count)
> > + size = count;
> > +
> > + /* Find which 4k chunk and offset with in that chunk */
> > + i = offset >> PAGE_CACHE_SHIFT;
> > + offset = offset & ~PAGE_CACHE_MASK;
> > +
> > + while (size) {
> > + chunksize = PAGE_CACHE_SIZE;
> > + if (offset)
> > + chunksize -= offset;
> > + if (chunksize > size)
> > + chunksize = size;
> > + kaddr = kmap(&page[i]);
> > + left = __copy_to_user(buf, kaddr + offset, chunksize);
> > + kunmap(&page[i]);
> > + if (left) {
> > + copied += (chunksize - left);
> > + break;
> > + }
> > + offset = 0;
> > + size -= chunksize;
> > + buf += chunksize;
> > + copied += chunksize;
> > + i++;
> > + }
> > + return copied ? copied : -EFAULT;
> > +}
>
> This returns -EFAULT when asked to read zero bytes. The caller prevents
> that, but it's a little bit ugly. Livable with.

I can fix that, but I didn't want to come here if length == 0 - so
took a shortcut.

>
> > +/*
> > + * Support for read() - Find the page attached to f_mapping and copy out the
> > + * data. Its *very* similar to do_generic_mapping_read(), we can't use that
> > + * since it has PAGE_CACHE_SIZE assumptions.
> > + */
> > +ssize_t
> > +hugetlbfs_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos)
> > +{
> > + struct address_space *mapping = filp->f_mapping;
> > + struct inode *inode = mapping->host;
> > + unsigned long index = *ppos >> HPAGE_SHIFT;
> > + unsigned long end_index;
> > + loff_t isize;
> > + unsigned long offset;
> > + ssize_t retval = 0;
> > +
> > + /* validate length */
> > + if (len == 0)
> > + goto out;
> > +
> > + isize = i_size_read(inode);
> > + if (!isize)
> > + goto out;
> > +
> > + offset = *ppos & ~HPAGE_MASK;
> > + end_index = (isize - 1) >> HPAGE_SHIFT;
> > + for (;;) {
> > + struct page *page;
> > + int nr, ret;
> > +
> > + /* nr is the maximum number of bytes to copy from this page */
> > + nr = HPAGE_SIZE;
> > + if (index >= end_index) {
> > + if (index > end_index)
> > + goto out;
> > + nr = ((isize - 1) & ~HPAGE_MASK) + 1;
> > + if (nr <= offset) {
> > + goto out;
> > + }
> > + }
> > + nr = nr - offset;
> > +
> > + /* Find the page */
> > + page = find_get_page(mapping, index);
> > + if (unlikely(page == NULL)) {
> > + /*
> > + * We can't find the page in the cache - bail out ?
> > + */
> > + goto out;
> > + }
> > + /*
> > + * Ok, we have the page, copy it to user space buffer.
> > + */
> > + ret = hugetlbfs_read_actor(page, offset, buf, len, nr);
> > + if (ret < 0) {
> > + retval = retval ? : ret;
> > + goto out;
>
> Missing put_page().

Yes. Thanks for catching it.

>
> > + }
> > +
> > + offset += ret;
> > + retval += ret;
> > + len -= ret;
> > + index += offset >> HPAGE_SHIFT;
> > + offset &= ~HPAGE_MASK;
> > +
> > + page_cache_release(page);
> > + if (ret == nr && len)
> > + continue;
> > + goto out;
> > + }
> > +out:
> > + return retval;
> > +}
>
> This code doesn't have all the ghastly tricks which we deploy to handle
> concurrent truncate.

Do I need to ? Baaahh!! I don't want to deal with them.
All I want is a simple read() to get my oprofile working.
Please advise.

Thanks,
Badari