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Thu, 10 Sep 2026 11:18:18 -0700 (PDT) Received: from daehojeong-desktop.mtv.corp.google.com ([2a00:79e0:2e7c:8:4a5b:802b:c0b:d1d7]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-336c2571039sm32309407eec.25.2026.09.10.11.18.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Sep 2026 11:18:17 -0700 (PDT) From: Daeho Jeong To: linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com Cc: Daeho Jeong Subject: [PATCH] f2fs: introduce reserve_shrink mount option for filesystem shrinkage Date: Thu, 10 Sep 2026 11:10:56 -0700 Message-ID: <20260910181057.640203-1-daeho43@gmail.com> X-Mailer: git-send-email 2.55.0.1007.g17ff1f9808-goog Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Daeho Jeong When preparing for subsequent online filesystem shrinkage (e.g., during partition resizing or FOTA updates), sufficient free space must be preserved so that valid data blocks can be evacuated and the filesystem can safely shrink. Existing reserve_root cannot prevent space exhaustion by privileged root processes (such as OTA updaters, package managers, and system daemons running with CAP_SYS_RESOURCE), which can allocate blocks from the root reserve and lead to resize failures due to lack of space. Moreover, runtime configurable reserved_blocks represents permanent GC and metadata headroom that must persist after resize, which would cause double-counting if inflated for shrinkage. To resolve this, introduce a dedicated reserve_shrink= mount option: 1. Symmetrically mirrors reserve_root= in block units to pre-reserve space specifically for subsequent filesystem shrinkage. 2. In get_available_block_count(), unconditionally deducts reserve_shrink_blocks for all callers, strictly rejecting all allocations (including root / CAP_SYS_RESOURCE) once available blocks are exhausted. 3. In f2fs_statfs(), deducts reserve_shrink_blocks from f_bfree (and f_bavail) so that filesystem statistics accurately reflect usable space. 4. In f2fs_resize_fs(), automatically resets reserve_shrink_blocks to 0 and clears F2FS_MOUNT_RESERVE_SHRINK upon successful shrink completion, releasing any remaining reservation. 5. In sysfs: reserved_blocks, subtracts reserve_shrink_blocks when validating the upper limit of configurable reserved blocks. Signed-off-by: Daeho Jeong --- Documentation/filesystems/f2fs.rst | 7 +++++ fs/f2fs/f2fs.h | 9 ++++++ fs/f2fs/gc.c | 5 ++++ fs/f2fs/super.c | 47 ++++++++++++++++++++++++++++++ fs/f2fs/sysfs.c | 13 +++++++-- 5 files changed, 79 insertions(+), 2 deletions(-) diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst index 771216f45207..dbdc5d5a83ad 100644 --- a/Documentation/filesystems/f2fs.rst +++ b/Documentation/filesystems/f2fs.rst @@ -191,6 +191,13 @@ reserve_node=%d Support configuring reserved nodes which are used for gid, the default limit is 12.5% of all nodes. resuid=%d The user ID which may use the reserved blocks and nodes. resgid=%d The group ID which may use the reserved blocks and nodes. +reserve_shrink=%d Support pre-reserving space for subsequent filesystem + shrinkage (e.g. during partition resizing or FOTA), + unit: blocks. Unlike reserve_root, allocations from + this reserved space strictly reject all callers + (including root / CAP_SYS_RESOURCE). Once the + filesystem is successfully shrunk via resize, this + value is automatically reset to 0. fault_injection=%d Enable fault injection in all supported types with specified injection rate. fault_type=%d Support configuring fault injection type, should be diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h index 62efc25cd107..c21ed3eacc6f 100644 --- a/fs/f2fs/f2fs.h +++ b/fs/f2fs/f2fs.h @@ -145,6 +145,7 @@ enum f2fs_mount_opt { */ F2FS_MOUNT_LAZYTIME, F2FS_MOUNT_RESERVE_NODE, + F2FS_MOUNT_RESERVE_SHRINK, }; #define F2FS_OPTION(sbi) ((sbi)->mount_opt) @@ -226,6 +227,7 @@ struct f2fs_mount_info { unsigned long long opt; block_t root_reserved_blocks; /* root reserved blocks */ block_t root_reserved_nodes; /* root reserved nodes */ + block_t reserve_shrink_blocks; /* reserve blocks for shrink */ kuid_t s_resuid; /* reserved blocks for uid */ kgid_t s_resgid; /* reserved blocks for gid */ int active_logs; /* # of active logs */ @@ -2658,6 +2660,13 @@ static inline unsigned int get_available_block_count(struct f2fs_sb_info *sbi, if (test_opt(sbi, RESERVE_ROOT) && !__allow_reserved_root(sbi, inode, cap)) avail_user_block_count -= F2FS_OPTION(sbi).root_reserved_blocks; + if (test_opt(sbi, RESERVE_SHRINK)) { + if (avail_user_block_count > F2FS_OPTION(sbi).reserve_shrink_blocks) + avail_user_block_count -= F2FS_OPTION(sbi).reserve_shrink_blocks; + else + avail_user_block_count = 0; + } + if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED))) { if (avail_user_block_count > sbi->unusable_block_count) avail_user_block_count -= sbi->unusable_block_count; diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c index bc22dde1cb30..556c4793478d 100644 --- a/fs/f2fs/gc.c +++ b/fs/f2fs/gc.c @@ -2492,6 +2492,11 @@ int f2fs_resize_fs(struct file *filp, __u64 block_count) spin_lock(&sbi->stat_lock); sbi->user_block_count += shrunk_blocks; spin_unlock(&sbi->stat_lock); + } else if (test_opt(sbi, RESERVE_SHRINK)) { + spin_lock(&sbi->stat_lock); + F2FS_OPTION(sbi).reserve_shrink_blocks = 0; + clear_opt(sbi, RESERVE_SHRINK); + spin_unlock(&sbi->stat_lock); } out_err: f2fs_up_write_trace(&sbi->cp_global_sem, &clc); diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c index f96abecb3c55..f7f82a32f99f 100644 --- a/fs/f2fs/super.c +++ b/fs/f2fs/super.c @@ -196,6 +196,7 @@ enum { Opt_data_flush, Opt_reserve_root, Opt_reserve_node, + Opt_reserve_shrink, Opt_resgid, Opt_resuid, Opt_mode, @@ -328,6 +329,7 @@ static const struct fs_parameter_spec f2fs_param_specs[] = { fsparam_flag("data_flush", Opt_data_flush), fsparam_u32("reserve_root", Opt_reserve_root), fsparam_u32("reserve_node", Opt_reserve_node), + fsparam_u32("reserve_shrink", Opt_reserve_shrink), fsparam_gid("resgid", Opt_resgid), fsparam_uid("resuid", Opt_resuid), fsparam_enum("mode", Opt_mode, f2fs_param_mode), @@ -407,6 +409,7 @@ static match_table_t f2fs_checkpoint_tokens = { #define F2FS_SPEC_lookup_mode (1 << 24) #define F2FS_SPEC_reserve_node (1 << 25) #define F2FS_SPEC_resizable_tail_secno (1 << 26) +#define F2FS_SPEC_reserve_shrink (1 << 27) struct f2fs_fs_context { struct f2fs_mount_info info; @@ -538,6 +541,27 @@ static inline void limit_reserve_root(struct f2fs_sb_info *sbi) F2FS_OPTION(sbi).s_resgid)); } +static inline void limit_reserve_shrink(struct f2fs_sb_info *sbi) +{ + block_t block_limit; + + if (!test_opt(sbi, RESERVE_SHRINK)) + return; + + block_limit = sbi->user_block_count - sbi->reserved_blocks; + if (test_opt(sbi, RESERVE_ROOT)) { + if (block_limit > F2FS_OPTION(sbi).root_reserved_blocks) + block_limit -= F2FS_OPTION(sbi).root_reserved_blocks; + else + block_limit = 0; + } + if (F2FS_OPTION(sbi).reserve_shrink_blocks > block_limit) { + F2FS_OPTION(sbi).reserve_shrink_blocks = block_limit; + f2fs_info(sbi, "Reduce reserved blocks for shrink = %u", + F2FS_OPTION(sbi).reserve_shrink_blocks); + } +} + static inline void adjust_unusable_cap_perc(struct f2fs_sb_info *sbi) { if (!F2FS_OPTION(sbi).unusable_cap_perc) @@ -941,6 +965,14 @@ static int f2fs_parse_param(struct fs_context *fc, struct fs_parameter *param) F2FS_CTX_INFO(ctx).root_reserved_nodes = result.uint_32; ctx->spec_mask |= F2FS_SPEC_reserve_node; break; + case Opt_reserve_shrink: + if (result.uint_32) + ctx_set_opt(ctx, F2FS_MOUNT_RESERVE_SHRINK); + else + ctx_clear_opt(ctx, F2FS_MOUNT_RESERVE_SHRINK); + F2FS_CTX_INFO(ctx).reserve_shrink_blocks = result.uint_32; + ctx->spec_mask |= F2FS_SPEC_reserve_shrink; + break; case Opt_resuid: F2FS_CTX_INFO(ctx).s_resuid = result.uid; ctx->spec_mask |= F2FS_SPEC_resuid; @@ -1763,6 +1795,9 @@ static void f2fs_apply_options(struct fs_context *fc, struct super_block *sb) if (ctx->spec_mask & F2FS_SPEC_reserve_node) F2FS_OPTION(sbi).root_reserved_nodes = F2FS_CTX_INFO(ctx).root_reserved_nodes; + if (ctx->spec_mask & F2FS_SPEC_reserve_shrink) + F2FS_OPTION(sbi).reserve_shrink_blocks = + F2FS_CTX_INFO(ctx).reserve_shrink_blocks; if (ctx->spec_mask & F2FS_SPEC_resgid) F2FS_OPTION(sbi).s_resgid = F2FS_CTX_INFO(ctx).s_resgid; if (ctx->spec_mask & F2FS_SPEC_resuid) @@ -2288,6 +2323,13 @@ static int f2fs_statfs(struct dentry *dentry, struct kstatfs *buf) buf->f_bfree = user_block_count - valid_user_blocks(sbi) - sbi->current_reserved_blocks; + if (test_opt(sbi, RESERVE_SHRINK)) { + if (buf->f_bfree > F2FS_OPTION(sbi).reserve_shrink_blocks) + buf->f_bfree -= F2FS_OPTION(sbi).reserve_shrink_blocks; + else + buf->f_bfree = 0; + } + if (unlikely(buf->f_bfree <= sbi->unusable_block_count)) buf->f_bfree = 0; else @@ -2512,6 +2554,9 @@ static int f2fs_show_options(struct seq_file *seq, struct dentry *root) F2FS_OPTION(sbi).s_resuid), from_kgid_munged(&init_user_ns, F2FS_OPTION(sbi).s_resgid)); + if (test_opt(sbi, RESERVE_SHRINK)) + seq_printf(seq, ",reserve_shrink=%u", + F2FS_OPTION(sbi).reserve_shrink_blocks); #ifdef CONFIG_F2FS_FAULT_INJECTION if (test_opt(sbi, FAULT_INJECTION)) { seq_printf(seq, ",fault_injection=%u", @@ -3093,6 +3138,7 @@ static int __f2fs_remount(struct fs_context *fc, struct super_block *sb) adjust_pinned_area_boundary(sbi); limit_reserve_root(sbi); + limit_reserve_shrink(sbi); fc->sb_flags = (flags & ~SB_LAZYTIME) | (sb->s_flags & SB_LAZYTIME); sbi->umount_lock_holder = NULL; @@ -5310,6 +5356,7 @@ static int f2fs_fill_super(struct super_block *sb, struct fs_context *fc) sbi->current_reserved_blocks = 0; sbi->alias_reserved_blocks = 0; limit_reserve_root(sbi); + limit_reserve_shrink(sbi); adjust_unusable_cap_perc(sbi); f2fs_init_extent_cache_info(sbi); diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c index aaca9ed9b169..d61940d095b4 100644 --- a/fs/f2fs/sysfs.c +++ b/fs/f2fs/sysfs.c @@ -591,9 +591,18 @@ static ssize_t __sbi_store(struct f2fs_attr *a, } #endif if (a->struct_type == RESERVED_BLOCKS) { + unsigned long limit; + spin_lock(&sbi->stat_lock); - if (t > (unsigned long)(sbi->user_block_count - - F2FS_OPTION(sbi).root_reserved_blocks)) { + limit = sbi->user_block_count - + F2FS_OPTION(sbi).root_reserved_blocks; + if (test_opt(sbi, RESERVE_SHRINK)) { + if (limit > F2FS_OPTION(sbi).reserve_shrink_blocks) + limit -= F2FS_OPTION(sbi).reserve_shrink_blocks; + else + limit = 0; + } + if (t > limit) { spin_unlock(&sbi->stat_lock); return -EINVAL; } -- 2.55.0.1007.g17ff1f9808-goog