smb: improve directory cache reuse for readdir operations

Currently, cached directory contents were not reused across subsequent
'ls' operations because the cache validity check relied on comparing
the ctx pointer, which changes with each readdir invocation. As a
result, the cached dir entries was not marked as valid and the cache was
not utilized for subsequent 'ls' operations.

This change uses the file pointer, which remains consistent across all
readdir calls for a given directory instance, to associate and validate
the cache. As a result, cached directory contents can now be
correctly reused, improving performance for repeated directory listings.

Performance gains with local windows SMB server:

Without the patch and default actimeo=1:
1000 directory enumeration operations on dir with 10k files took 135.0s

With this patch and actimeo=0:
1000 directory enumeration operations on dir with 10k files took just 5.1s

Signed-off-by: Bharath SM <[e-mail]>
Reviewed-by: Shyam Prasad N <[e-mail]>
Cc: [e-mail]
Signed-off-by: Steve French <[e-mail]>

fs/smb/client/cached_dir.h

···
 struct cached_dirents {
 	bool is_valid:1;
 	bool is_failed:1;
-	struct dir_context *ctx; /*
-				  * Only used to make sure we only take entries
-				  * from a single context. Never dereferenced.
-				  */
+	struct file *file; /*
+			    * Used to associate the cache with a single
+			    * open file instance.
+			    */
 	struct mutex de_mutex;
 	int pos;		 /* Expected ctx->pos */
 	struct list_head entries;

fs/smb/client/readdir.c

···
 }
 
 static void update_cached_dirents_count(struct cached_dirents *cde,
-					struct dir_context *ctx)
+					struct file *file)
 {
-	if (cde->ctx != ctx)
+	if (cde->file != file)
 		return;
 	if (cde->is_valid || cde->is_failed)
 		return;
···
 }
 
 static void finished_cached_dirents_count(struct cached_dirents *cde,
-					struct dir_context *ctx)
+					struct dir_context *ctx, struct file *file)
 {
-	if (cde->ctx != ctx)
+	if (cde->file != file)
 		return;
 	if (cde->is_valid || cde->is_failed)
 		return;
···
 static void add_cached_dirent(struct cached_dirents *cde,
 			      struct dir_context *ctx,
 			      const char *name, int namelen,
-			      struct cifs_fattr *fattr)
+			      struct cifs_fattr *fattr,
+				  struct file *file)
 {
 	struct cached_dirent *de;
 
-	if (cde->ctx != ctx)
+	if (cde->file != file)
 		return;
 	if (cde->is_valid || cde->is_failed)
 		return;
···
 static bool cifs_dir_emit(struct dir_context *ctx,
 			  const char *name, int namelen,
 			  struct cifs_fattr *fattr,
-			  struct cached_fid *cfid)
+			  struct cached_fid *cfid,
+			  struct file *file)
 {
 	bool rc;
 	ino_t ino = cifs_uniqueid_to_ino_t(fattr->cf_uniqueid);
···
 	if (cfid) {
 		mutex_lock(&cfid->dirents.de_mutex);
 		add_cached_dirent(&cfid->dirents, ctx, name, namelen,
-				  fattr);
+				  fattr, file);
 		mutex_unlock(&cfid->dirents.de_mutex);
 	}
 
···
 	cifs_prime_dcache(file_dentry(file), &name, &fattr);
 
 	return !cifs_dir_emit(ctx, name.name, name.len,
-			      &fattr, cfid);
+			      &fattr, cfid, file);
 }
 
 
···
 	 * we need to initialize scanning and storing the
 	 * directory content.
 	 */
-	if (ctx->pos == 0 && cfid->dirents.ctx == NULL) {
-		cfid->dirents.ctx = ctx;
+	if (ctx->pos == 0 && cfid->dirents.file == NULL) {
+		cfid->dirents.file = file;
 		cfid->dirents.pos = 2;
 	}
 	/*
···
 	} else {
 		if (cfid) {
 			mutex_lock(&cfid->dirents.de_mutex);
-			finished_cached_dirents_count(&cfid->dirents, ctx);
+			finished_cached_dirents_count(&cfid->dirents, ctx, file);
 			mutex_unlock(&cfid->dirents.de_mutex);
 		}
 		cifs_dbg(FYI, "Could not find entry\n");
···
 		ctx->pos++;
 		if (cfid) {
 			mutex_lock(&cfid->dirents.de_mutex);
-			update_cached_dirents_count(&cfid->dirents, ctx);
+			update_cached_dirents_count(&cfid->dirents, file);
 			mutex_unlock(&cfid->dirents.de_mutex);
 		}