shmem_parse_options(): use a separate structure to keep the results

... and copy the data from it into sbinfo in the callers.
For use by remount we need to keep track whether there'd
been options setting max_inodes, max_blocks and huge resp.
and do the sanity checks (and copying) only if such options
had been seen.  uid/gid/mode is ignored by remount and
NULL mpol is already explicitly treated as "ignore it",
so we don't need to keep track of those.

Note: theoretically, mpol_parse_string() may return NULL
not in case of error (for default policy), so the assumption
that NULL mpol means "change nothing" is incorrect.  However,
that's the mainline behaviour and any changes belong in
a separate patch.  If we go for that, we'll need to keep
track of having encountered mpol= option too.

[changes in remount logics from Hugh Dickins folded]
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/shmem.c

@@ -107,6 +107,20 @@ struct shmem_falloc {
 	pgoff_t nr_unswapped;	/* how often writepage refused to swap out */
 };
 
+struct shmem_options {
+	unsigned long long blocks;
+	unsigned long long inodes;
+	struct mempolicy *mpol;
+	kuid_t uid;
+	kgid_t gid;
+	umode_t mode;
+	int huge;
+	int seen;
+#define SHMEM_SEEN_BLOCKS 1
+#define SHMEM_SEEN_INODES 2
+#define SHMEM_SEEN_HUGE 4
+};
+
 #ifdef CONFIG_TMPFS
 static unsigned long shmem_default_max_blocks(void)
 {
@@ -3349,8 +3363,7 @@ static const struct export_operations shmem_export_ops = {
 	.fh_to_dentry	= shmem_fh_to_dentry,
 };
 
-static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
-			       bool remount)
+static int shmem_parse_options(char *options, struct shmem_options *ctx)
 {
 	char *this_char, *value, *rest;
 	struct mempolicy *mpol = NULL;
@@ -3395,39 +3408,35 @@ static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
 			}
 			if (*rest)
 				goto bad_val;
-			sbinfo->max_blocks =
-				DIV_ROUND_UP(size, PAGE_SIZE);
+			ctx->blocks = DIV_ROUND_UP(size, PAGE_SIZE);
+			ctx->seen |= SHMEM_SEEN_BLOCKS;
 		} else if (!strcmp(this_char,"nr_blocks")) {
-			sbinfo->max_blocks = memparse(value, &rest);
+			ctx->blocks = memparse(value, &rest);
 			if (*rest)
 				goto bad_val;
+			ctx->seen |= SHMEM_SEEN_BLOCKS;
 		} else if (!strcmp(this_char,"nr_inodes")) {
-			sbinfo->max_inodes = memparse(value, &rest);
+			ctx->inodes = memparse(value, &rest);
 			if (*rest)
 				goto bad_val;
+			ctx->seen |= SHMEM_SEEN_INODES;
 		} else if (!strcmp(this_char,"mode")) {
-			if (remount)
-				continue;
-			sbinfo->mode = simple_strtoul(value, &rest, 8) & 07777;
+			ctx->mode = simple_strtoul(value, &rest, 8) & 07777;
 			if (*rest)
 				goto bad_val;
 		} else if (!strcmp(this_char,"uid")) {
-			if (remount)
-				continue;
 			uid = simple_strtoul(value, &rest, 0);
 			if (*rest)
 				goto bad_val;
-			sbinfo->uid = make_kuid(current_user_ns(), uid);
-			if (!uid_valid(sbinfo->uid))
+			ctx->uid = make_kuid(current_user_ns(), uid);
+			if (!uid_valid(ctx->uid))
 				goto bad_val;
 		} else if (!strcmp(this_char,"gid")) {
-			if (remount)
-				continue;
 			gid = simple_strtoul(value, &rest, 0);
 			if (*rest)
 				goto bad_val;
-			sbinfo->gid = make_kgid(current_user_ns(), gid);
-			if (!gid_valid(sbinfo->gid))
+			ctx->gid = make_kgid(current_user_ns(), gid);
+			if (!gid_valid(ctx->gid))
 				goto bad_val;
 #ifdef CONFIG_TRANSPARENT_HUGE_PAGECACHE
 		} else if (!strcmp(this_char, "huge")) {
@@ -3438,7 +3447,8 @@ static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
 			if (!has_transparent_hugepage() &&
 					huge != SHMEM_HUGE_NEVER)
 				goto bad_val;
-			sbinfo->huge = huge;
+			ctx->huge = huge;
+			ctx->seen |= SHMEM_SEEN_HUGE;
 #endif
 #ifdef CONFIG_NUMA
 		} else if (!strcmp(this_char,"mpol")) {
@@ -3452,7 +3462,7 @@ static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
 			goto error;
 		}
 	}
-	sbinfo->mpol = mpol;
+	ctx->mpol = mpol;
 	return 0;
 
 bad_val:
@@ -3467,42 +3477,50 @@ static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
 static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
 {
 	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
-	struct shmem_sb_info config = *sbinfo;
+	struct shmem_options ctx = {.seen = 0};
 	unsigned long inodes;
 	int error = -EINVAL;
 
-	config.mpol = NULL;
-	if (shmem_parse_options(data, &config, true))
+	if (shmem_parse_options(data, &ctx))
 		return error;
 
 	spin_lock(&sbinfo->stat_lock);
 	inodes = sbinfo->max_inodes - sbinfo->free_inodes;
-	if (percpu_counter_compare(&sbinfo->used_blocks, config.max_blocks) > 0)
-		goto out;
-	if (config.max_inodes < inodes)
-		goto out;
 	/*
 	 * Those tests disallow limited->unlimited while any are in use;
 	 * but we must separately disallow unlimited->limited, because
 	 * in that case we have no record of how much is already in use.
 	 */
-	if (config.max_blocks && !sbinfo->max_blocks)
-		goto out;
-	if (config.max_inodes && !sbinfo->max_inodes)
-		goto out;
+	if ((ctx.seen & SHMEM_SEEN_BLOCKS) && ctx.blocks) {
+		if (!sbinfo->max_blocks)
+			goto out;
+		if (percpu_counter_compare(&sbinfo->used_blocks,
+					   ctx.blocks) > 0)
+			goto out;
+	}
+	if ((ctx.seen & SHMEM_SEEN_INODES) && ctx.inodes) {
+		if (!sbinfo->max_inodes)
+			goto out;
+		if (ctx.inodes < inodes)
+			goto out;
+	}
 
 	error = 0;
-	sbinfo->huge = config.huge;
-	sbinfo->max_blocks  = config.max_blocks;
-	sbinfo->max_inodes  = config.max_inodes;
-	sbinfo->free_inodes = config.max_inodes - inodes;
+	if (ctx.seen & SHMEM_SEEN_HUGE)
+		sbinfo->huge = ctx.huge;
+	if (ctx.seen & SHMEM_SEEN_BLOCKS)
+		sbinfo->max_blocks  = ctx.blocks;
+	if (ctx.seen & SHMEM_SEEN_INODES) {
+		sbinfo->max_inodes  = ctx.inodes;
+		sbinfo->free_inodes = ctx.inodes - inodes;
+	}
 
 	/*
 	 * Preserve previous mempolicy unless mpol remount option was specified.
 	 */
-	if (config.mpol) {
+	if (ctx.mpol) {
 		mpol_put(sbinfo->mpol);
-		sbinfo->mpol = config.mpol;	/* transfers initial ref */
+		sbinfo->mpol = ctx.mpol;	/* transfers initial ref */
 	}
 out:
 	spin_unlock(&sbinfo->stat_lock);
@@ -3551,6 +3569,9 @@ static int shmem_fill_super(struct super_block *sb, void *data, int silent)
 {
 	struct inode *inode;
 	struct shmem_sb_info *sbinfo;
+	struct shmem_options ctx = {.mode = 0777 | S_ISVTX,
+				    .uid = current_fsuid(),
+				    .gid = current_fsgid()};
 	int err = -ENOMEM;
 
 	/* Round up to L1_CACHE_BYTES to resist false sharing */
@@ -3559,9 +3580,6 @@ static int shmem_fill_super(struct super_block *sb, void *data, int silent)
 	if (!sbinfo)
 		return -ENOMEM;
 
-	sbinfo->mode = 0777 | S_ISVTX;
-	sbinfo->uid = current_fsuid();
-	sbinfo->gid = current_fsgid();
 	sb->s_fs_info = sbinfo;
 
 #ifdef CONFIG_TMPFS
@@ -3571,9 +3589,9 @@ static int shmem_fill_super(struct super_block *sb, void *data, int silent)
 	 * but the internal instance is left unlimited.
 	 */
 	if (!(sb->s_flags & SB_KERNMOUNT)) {
-		sbinfo->max_blocks = shmem_default_max_blocks();
-		sbinfo->max_inodes = shmem_default_max_inodes();
-		if (shmem_parse_options(data, sbinfo, false)) {
+		ctx.blocks = shmem_default_max_blocks();
+		ctx.inodes = shmem_default_max_inodes();
+		if (shmem_parse_options(data, &ctx)) {
 			err = -EINVAL;
 			goto failed;
 		}
@@ -3585,11 +3603,17 @@ static int shmem_fill_super(struct super_block *sb, void *data, int silent)
 #else
 	sb->s_flags |= SB_NOUSER;
 #endif
+	sbinfo->max_blocks = ctx.blocks;
+	sbinfo->free_inodes = sbinfo->max_inodes = ctx.inodes;
+	sbinfo->uid = ctx.uid;
+	sbinfo->gid = ctx.gid;
+	sbinfo->mode = ctx.mode;
+	sbinfo->huge = ctx.huge;
+	sbinfo->mpol = ctx.mpol;
 
 	spin_lock_init(&sbinfo->stat_lock);
 	if (percpu_counter_init(&sbinfo->used_blocks, 0, GFP_KERNEL))
 		goto failed;
-	sbinfo->free_inodes = sbinfo->max_inodes;
 	spin_lock_init(&sbinfo->shrinklist_lock);
 	INIT_LIST_HEAD(&sbinfo->shrinklist);