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Subject[PATCH] hugetlbfs read() support
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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;
+}
+
+/*
+ * 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;
+ }
+
+ 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;
+}
+
/*
* Read a page. Again trivial. If it didn't already exist
* in the page cache, it is zero-filled.
@@ -560,6 +672,7 @@ static void init_once(void *foo, struct
}

const struct file_operations hugetlbfs_file_operations = {
+ .read = hugetlbfs_read,
.mmap = hugetlbfs_file_mmap,
.fsync = simple_sync_file,
.get_unmapped_area = hugetlb_get_unmapped_area,

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