erofs: get correct count for unmapped range in fscache mode

For unmapped range, the returned map.m_llen is zero, and thus the
calculated count is unexpected zero.

Prior to the refactoring introduced by commit 1ae9470c3e14 ("erofs:
clean up .read_folio() and .readahead() in fscache mode"), only the
readahead routine suffers from this. With the refactoring of making
.read_folio() and .readahead() calling one common routine, both
read_folio and readahead have this issue now.

Fix this by calculating count separately in unmapped condition.

Fixes: c665b394b9e8 ("erofs: implement fscache-based data readahead")
Fixes: 1ae9470c3e14 ("erofs: clean up .read_folio() and .readahead() in fscache mode")
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20221104054028.52208-3-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

diff --git a/fs/erofs/fscache.c b/fs/erofs/fscache.c
index 83559008bfa89085e32384bb0f0b60eb60cc85ad..260fa4737fc006b9b2d646d94a24ec37ea079e95 100644 (file)
--- a/fs/erofs/fscache.c
+++ b/fs/erofs/fscache.c
@@ -296,15 +296,16 @@ static int erofs_fscache_data_read(struct address_space *mapping,
                return PAGE_SIZE;
        }
 
-       count = min_t(size_t, map.m_llen - (pos - map.m_la), len);
-       DBG_BUGON(!count || count % PAGE_SIZE);
-
        if (!(map.m_flags & EROFS_MAP_MAPPED)) {
+               count = len;
                iov_iter_xarray(&iter, READ, &mapping->i_pages, pos, count);
                iov_iter_zero(count, &iter);
                return count;
        }
 
+       count = min_t(size_t, map.m_llen - (pos - map.m_la), len);
+       DBG_BUGON(!count || count % PAGE_SIZE);
+
        mdev = (struct erofs_map_dev) {
                .m_deviceid = map.m_deviceid,
                .m_pa = map.m_pa,