flexfiles/pNFS: fix NULL checks on result of ff_layout_choose_ds_for_read

JIRA: https://issues.redhat.com/browse/RHEL-110294

commit 5a46d23
Author: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Date:   Thu Aug 28 16:51:00 2025 +0200

    flexfiles/pNFS: fix NULL checks on result of ff_layout_choose_ds_for_read

    Recent commit f06bedf ("pNFS/flexfiles: don't attempt pnfs on fatal DS
    errors") has changed the error return type of ff_layout_choose_ds_for_read() from
    NULL to an error pointer. However, not all code paths have been updated
    to match the change. Thus, some non-NULL checks will accept error pointers
    as a valid return value.

    Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
    Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
    Fixes: f06bedf ("pNFS/flexfiles: don't attempt pnfs on fatal DS errors")
    Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
    Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>

Author: bcodding-rh

fs/nfs/flexfilelayout/flexfilelayout.c

@@ -773,8 +773,11 @@ ff_layout_choose_ds_for_read(struct pnfs_layout_segment *lseg,
 			continue;
 
 		if (check_device &&
-		    nfs4_test_deviceid_unavailable(&mirror->mirror_ds->id_node))
+		    nfs4_test_deviceid_unavailable(&mirror->mirror_ds->id_node)) {
+			// reinitialize the error state in case if this is the last iteration
+			ds = ERR_PTR(-EINVAL);
 			continue;
+		}
 
 		*best_idx = idx;
 		break;
@@ -804,7 +807,7 @@ ff_layout_choose_best_ds_for_read(struct pnfs_layout_segment *lseg,
 	struct nfs4_pnfs_ds *ds;
 
 	ds = ff_layout_choose_valid_ds_for_read(lseg, start_idx, best_idx);
-	if (ds)
+	if (!IS_ERR(ds))
 		return ds;
 	return ff_layout_choose_any_ds_for_read(lseg, start_idx, best_idx);
 }
@@ -818,7 +821,7 @@ ff_layout_get_ds_for_read(struct nfs_pageio_descriptor *pgio,
 
 	ds = ff_layout_choose_best_ds_for_read(lseg, pgio->pg_mirror_idx,
 					       best_idx);
-	if (ds || !pgio->pg_mirror_idx)
+	if (!IS_ERR(ds) || !pgio->pg_mirror_idx)
 		return ds;
 	return ff_layout_choose_best_ds_for_read(lseg, 0, best_idx);
 }
@@ -868,7 +871,7 @@ ff_layout_pg_init_read(struct nfs_pageio_descriptor *pgio,
 	req->wb_nio = 0;
 
 	ds = ff_layout_get_ds_for_read(pgio, &ds_idx);
-	if (!ds) {
+	if (IS_ERR(ds)) {
 		if (!ff_layout_no_fallback_to_mds(pgio->pg_lseg))
 			goto out_mds;
 		pnfs_generic_pg_cleanup(pgio);
@@ -1072,11 +1075,13 @@ static void ff_layout_resend_pnfs_read(struct nfs_pgio_header *hdr)
 {
 	u32 idx = hdr->pgio_mirror_idx + 1;
 	u32 new_idx = 0;
+	struct nfs4_pnfs_ds *ds;
 
-	if (ff_layout_choose_any_ds_for_read(hdr->lseg, idx, &new_idx))
-		ff_layout_send_layouterror(hdr->lseg);
-	else
+	ds = ff_layout_choose_any_ds_for_read(hdr->lseg, idx, &new_idx);
+	if (IS_ERR(ds))
 		pnfs_error_mark_layout_for_return(hdr->inode, hdr->lseg);
+	else
+		ff_layout_send_layouterror(hdr->lseg);
 	pnfs_read_resend_pnfs(hdr, new_idx);
 }