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Subject[patch 2/3] Use an atomic kmap for generic_file_read()


This patch allows the kernel to hold atomic kmaps in file_read_actor().

We try to fault in the page, then take an atomic kmap. If the atomic
copy_to_user() then faults, drop a printk and fall back to kmap().



filemap.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++--
1 files changed, 47 insertions(+), 2 deletions(-)

--- 2.5.33/mm/filemap.c~kmap_atomic_reads Thu Sep 5 00:58:46 2002
+++ 2.5.33-akpm/mm/filemap.c Thu Sep 5 00:58:46 2002
@@ -1036,7 +1036,37 @@ no_cached_page:
UPDATE_ATIME(inode);
}

-int file_read_actor(read_descriptor_t * desc, struct page *page, unsigned long offset, unsigned long size)
+/*
+ * Fault a userspace page into pagetables. Return non-zero on a fault.
+ *
+ * FIXME: this assumes that two userspace pages are always sufficient. That's
+ * not true if PAGE_CACHE_SIZE > PAGE_SIZE.
+ */
+static inline int fault_in_pages_writeable(char *uaddr, int size)
+{
+ int ret;
+
+ /*
+ * Writing zeroes into userspace here is OK, because we know that if
+ * the zero gets there, we'll be overwriting it.
+ */
+ ret = __put_user(0, uaddr);
+ if (ret == 0) {
+ char *end = uaddr + size - 1;
+
+ /*
+ * If the page was already mapped, this will get a cache miss
+ * for sure, so try to avoid doing it.
+ */
+ if (((unsigned long)uaddr & PAGE_MASK) !=
+ ((unsigned long)end & PAGE_MASK))
+ ret = __put_user(0, end);
+ }
+ return ret;
+}
+
+int file_read_actor(read_descriptor_t *desc, struct page *page,
+ unsigned long offset, unsigned long size)
{
char *kaddr;
unsigned long left, count = desc->count;
@@ -1044,14 +1074,29 @@ int file_read_actor(read_descriptor_t *
if (size > count)
size = count;

+ /*
+ * Faults on the destination of a read are common, so do it before
+ * taking the kmap.
+ */
+ if (!fault_in_pages_writeable(desc->buf, size)) {
+ kaddr = kmap_atomic(page, KM_USER0);
+ left = __copy_to_user(desc->buf, kaddr + offset, size);
+ kunmap_atomic(kaddr, KM_USER0);
+ if (left == 0)
+ goto success;
+ printk("%s: Unexpected page fault\n", __FUNCTION__);
+ }
+
+ /* Do it the slow way */
kaddr = kmap(page);
left = __copy_to_user(desc->buf, kaddr + offset, size);
kunmap(page);
-
+
if (left) {
size -= left;
desc->error = -EFAULT;
}
+success:
desc->count = count - size;
desc->written += size;
desc->buf += size;
.
-
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