uiomove_object_page(): Avoid instantiating pages in sparse regions on reads.

Check whether the page being requested is either resident or on swap.  If
not, read from the zero_region instead of instantiating an unnecessary page.

This avoids consuming memory for sparse files on tmpfs, when they are read
by applications that do not use SEEK_HOLE/SEEK_DATA (which is most of them).

Reviewed by:	kib
MFC after:	1 week
Sponsored by:	Spectra Logic

1 files changed, 11 insertions, 0 deletions

diff --git a/sys/kern/uipc_shm.c b/sys/kern/uipc_shm.c
index 14347d11602f..8410ed9b894a 100644
--- a/sys/kern/uipc_shm.c
+++ b/sys/kern/uipc_shm.c
@@ -163,6 +163,17 @@ uiomove_object_page(vm_object_t obj, size_t len, struct uio *uio)
 	VM_OBJECT_WLOCK(obj);
 
 	/*
+	 * Read I/O without either a corresponding resident page or swap
+	 * page: use zero_region.  This is intended to avoid instantiating
+	 * pages on read from a sparse region.
+	 */
+	if (uio->uio_rw == UIO_READ && vm_page_lookup(obj, idx) == NULL &&
+	    !vm_pager_has_page(obj, idx, NULL, NULL)) {
+		VM_OBJECT_WUNLOCK(obj);
+		return (uiomove(__DECONST(void *, zero_region), len, uio));
+	}
+
+	/*
 	 * Parallel reads of the page content from disk are prevented
 	 * by exclusive busy.
 	 *