[PATCH] NFS: restore directory index when resuming readdir

[Avi Weiss <thnkslprpt@gmail.com>]

nfs_readdir() restores the cached folio index and cookie when
resuming directory iteration, but leaves current_index at zero.
In 32-bit index mode, nfs_readdir_search_for_cookie() then
reports a folio-relative offset instead of the absolute entry
position.

Restore current_index from the saved logical folio index before
searching the cache.

This solution relies on the current behavior of 
nfs_readdir_seek_next_array(), where folio_index advances by 
one and current_index advances by nfs_readdir_array_maxentries()
for each full, nonterminal folio.

Fixes: f648022faa68e ("NFS: Convert readdir page cache to use a cookie based index")
Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>
---
I tried a couple other ways of recovering current_index like 
tracking it with a new variable in nfs_open_dir_context and 
deriving it from ctx->pos but the first option ended up expanding
the change-set drastically and the 2nd option had an issue of 
freshness after directory changes.

I was able to reproduce both the failure and confirm the fix is valid
locally.

 fs/nfs/dir.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 49394123bd09..a1ad04b2f307 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1253,6 +1253,8 @@ static int nfs_readdir(struct file *file, struct dir_context *ctx)
 	spin_lock(&file->f_lock);
 	desc->dir_cookie = dir_ctx->dir_cookie;
 	desc->folio_index = dir_ctx->page_index;
+	desc->current_index = (loff_t)desc->folio_index *
+			      nfs_readdir_array_maxentries();
 	desc->last_cookie = dir_ctx->last_cookie;
 	desc->attr_gencount = dir_ctx->attr_gencount;
 	desc->eof = dir_ctx->eof;

base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222
-- 
2.43.0