shmem: add support to ignore swap

In doing experimentations with shmem having the option to avoid swap
becomes a useful mechanism.  One of the *raves* about brd over shmem is
you can avoid swap, but that's not really a good reason to use brd if we
can instead use shmem.  Using brd has its own good reasons to exist, but
just because "tmpfs" doesn't let you do that is not a great reason to
avoid it if we can easily add support for it.

I don't add support for reconfiguring incompatible options, but if we
really wanted to we can add support for that.

To avoid swap we use mapping_set_unevictable() upon inode creation, and
put a WARN_ON_ONCE() stop-gap on writepages() for reclaim.

Link: https://lkml.kernel.org/r/20230309230545.2930737-7-mcgrof@kernel.org
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Christian Brauner <brauner@kernel.org>
Tested-by: Xin Hao <xhao@linux.alibaba.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Adam Manzanares <a.manzanares@samsung.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 27 insertions, 1 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 7751c136eadb..787e83791eb5 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -116,10 +116,12 @@ struct shmem_options {
 	bool full_inums;
 	int huge;
 	int seen;
+	bool noswap;
 #define SHMEM_SEEN_BLOCKS 1
 #define SHMEM_SEEN_INODES 2
 #define SHMEM_SEEN_HUGE 4
 #define SHMEM_SEEN_INUMS 8
+#define SHMEM_SEEN_NOSWAP 16
 };
 
 #ifdef CONFIG_TMPFS
@@ -1334,6 +1336,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	struct address_space *mapping = folio->mapping;
 	struct inode *inode = mapping->host;
 	struct shmem_inode_info *info = SHMEM_I(inode);
+	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 	swp_entry_t swap;
 	pgoff_t index;
 
@@ -1347,7 +1350,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	if (WARN_ON_ONCE(!wbc->for_reclaim))
 		goto redirty;
 
-	if (WARN_ON_ONCE(info->flags & VM_LOCKED))
+	if (WARN_ON_ONCE((info->flags & VM_LOCKED) || sbinfo->noswap))
 		goto redirty;
 
 	if (!total_swap_pages)
@@ -2372,6 +2375,8 @@ static struct inode *shmem_get_inode(struct mnt_idmap *idmap, struct super_block
 			shmem_set_inode_flags(inode, info->fsflags);
 		INIT_LIST_HEAD(&info->shrinklist);
 		INIT_LIST_HEAD(&info->swaplist);
+		if (sbinfo->noswap)
+			mapping_set_unevictable(inode->i_mapping);
 		simple_xattrs_init(&info->xattrs);
 		cache_no_acl(inode);
 		mapping_set_large_folios(inode->i_mapping);
@@ -3459,6 +3464,7 @@ enum shmem_param {
 	Opt_uid,
 	Opt_inode32,
 	Opt_inode64,
+	Opt_noswap,
 };
 
 static const struct constant_table shmem_param_enums_huge[] = {
@@ -3480,6 +3486,7 @@ const struct fs_parameter_spec shmem_fs_parameters[] = {
 	fsparam_u32   ("uid",		Opt_uid),
 	fsparam_flag  ("inode32",	Opt_inode32),
 	fsparam_flag  ("inode64",	Opt_inode64),
+	fsparam_flag  ("noswap",	Opt_noswap),
 	{}
 };
 
@@ -3563,6 +3570,10 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
 		ctx->full_inums = true;
 		ctx->seen |= SHMEM_SEEN_INUMS;
 		break;
+	case Opt_noswap:
+		ctx->noswap = true;
+		ctx->seen |= SHMEM_SEEN_NOSWAP;
+		break;
 	}
 	return 0;
 
@@ -3661,6 +3672,14 @@ static int shmem_reconfigure(struct fs_context *fc)
 		err = "Current inum too high to switch to 32-bit inums";
 		goto out;
 	}
+	if ((ctx->seen & SHMEM_SEEN_NOSWAP) && ctx->noswap && !sbinfo->noswap) {
+		err = "Cannot disable swap on remount";
+		goto out;
+	}
+	if (!(ctx->seen & SHMEM_SEEN_NOSWAP) && !ctx->noswap && sbinfo->noswap) {
+		err = "Cannot enable swap on remount if it was disabled on first mount";
+		goto out;
+	}
 
 	if (ctx->seen & SHMEM_SEEN_HUGE)
 		sbinfo->huge = ctx->huge;
@@ -3681,6 +3700,10 @@ static int shmem_reconfigure(struct fs_context *fc)
 		sbinfo->mpol = ctx->mpol;	/* transfers initial ref */
 		ctx->mpol = NULL;
 	}
+
+	if (ctx->noswap)
+		sbinfo->noswap = true;
+
 	raw_spin_unlock(&sbinfo->stat_lock);
 	mpol_put(mpol);
 	return 0;
@@ -3735,6 +3758,8 @@ static int shmem_show_options(struct seq_file *seq, struct dentry *root)
 		seq_printf(seq, ",huge=%s", shmem_format_huge(sbinfo->huge));
 #endif
 	shmem_show_mpol(seq, sbinfo->mpol);
+	if (sbinfo->noswap)
+		seq_printf(seq, ",noswap");
 	return 0;
 }
 
@@ -3778,6 +3803,7 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
 			ctx->inodes = shmem_default_max_inodes();
 		if (!(ctx->seen & SHMEM_SEEN_INUMS))
 			ctx->full_inums = IS_ENABLED(CONFIG_TMPFS_INODE64);
+		sbinfo->noswap = ctx->noswap;
 	} else {
 		sb->s_flags |= SB_NOUSER;
 	}