CacheFiles: Don't write a full page if there's only a partial page to cache

cachefiles_write_page() writes a full page to the backing file for the last
page of the netfs file, even if the netfs file's last page is only a partial
page.

This causes the EOF on the backing file to be extended beyond the EOF of the
netfs, and thus the backing file will be truncated by cachefiles_attr_changed()
called from cachefiles_lookup_object().

So we need to limit the write we make to the backing file on that last page
such that it doesn't push the EOF too far.

Also, if a backing file that has a partial page at the end is expanded, we
discard the partial page and refetch it on the basis that we then have a hole
in the file with invalid data, and should the power go out...  A better way to
deal with this could be to record a note that the partial page contains invalid
data until the correct data is written into it.

This isn't a problem for netfs's that discard the whole backing file if the
file size changes (such as NFS).

Signed-off-by: David Howells <dhowells@redhat.com>

2 files changed, 36 insertions, 7 deletions

diff --git a/fs/cachefiles/interface.c b/fs/cachefiles/interface.c
index dd7f852746c..8e67abf0598 100644
--- a/fs/cachefiles/interface.c
+++ b/fs/cachefiles/interface.c
@@ -404,12 +404,26 @@ static int cachefiles_attr_changed(struct fscache_object *_object)
         if (oi_size == ni_size)
                 return 0;
 
-        newattrs.ia_size = ni_size;
-        newattrs.ia_valid = ATTR_SIZE;
-
         cachefiles_begin_secure(cache, &saved_cred);
         mutex_lock(&object->backer->d_inode->i_mutex);
+
+        /* if there's an extension to a partial page at the end of the backing
+         * file, we need to discard the partial page so that we pick up new
+         * data after it */
+        if (oi_size & ~PAGE_MASK && ni_size > oi_size) {
+                _debug("discard tail %llx", oi_size);
+                newattrs.ia_valid = ATTR_SIZE;
+                newattrs.ia_size = oi_size & PAGE_MASK;
+                ret = notify_change(object->backer, &newattrs);
+                if (ret < 0)
+                        goto truncate_failed;
+        }
+
+        newattrs.ia_valid = ATTR_SIZE;
+        newattrs.ia_size = ni_size;
         ret = notify_change(object->backer, &newattrs);
+
+truncate_failed:
         mutex_unlock(&object->backer->d_inode->i_mutex);
         cachefiles_end_secure(cache, saved_cred);
 
diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
index 3304646dae8..5a84fd7109a 100644
--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -803,7 +803,8 @@ int cachefiles_write_page(struct fscache_storage *op, struct page *page)
         struct cachefiles_cache *cache;
         mm_segment_t old_fs;
         struct file *file;
-        loff_t pos;
+        loff_t pos, eof;
+        size_t len;
         void *data;
         int ret;
 
@@ -837,15 +838,29 @@ int cachefiles_write_page(struct fscache_storage *op, struct page *page)
                 ret = -EIO;
                 if (file->f_op->write) {
                         pos = (loff_t) page->index << PAGE_SHIFT;
+
+                        /* we mustn't write more data than we have, so we have
+                         * to beware of a partial page at EOF */
+                        eof = object->fscache.store_limit_l;
+                        len = PAGE_SIZE;
+                        if (eof & ~PAGE_MASK) {
+                                ASSERTCMP(pos, <, eof);
+                                if (eof - pos < PAGE_SIZE) {
+                                        _debug("cut short %llx to %llx",
+                                               pos, eof);
+                                        len = eof - pos;
+                                        ASSERTCMP(pos + len, ==, eof);
+                                }
+                        }
+
                         data = kmap(page);
                         old_fs = get_fs();
                         set_fs(KERNEL_DS);
                         ret = file->f_op->write(
-                                file, (const void __user *) data, PAGE_SIZE,
-                                &pos);
+                                file, (const void __user *) data, len, &pos);
                         set_fs(old_fs);
                         kunmap(page);
-                        if (ret != PAGE_SIZE)
+                        if (ret != len)
                                 ret = -EIO;
                 }
                 fput(file);