[PATCH v4 09/16] erofs: use folio_attach/detach_private() instead of direct assignment
flat view
COLD23d
REVIEWED: 1 (1M)
From: Zi Yan <ziy@nvidia.com>
Date: 2026-09-14 02:25:11
Also in:
lkml
Subsystem:
erofs file system, filesystems (vfs and infrastructure), the rest · Maintainers:
Gao Xiang, Chao Yu, Alexander Viro, Christian Brauner, Linus Torvalds
1 review trailer (1 from subsystem maintainers).
erofs_onlinefolio_init/split/end() use folio->private without setting PG_private or increasing folio refcount and it works. But after PG_private is replaced by checking folio->private in a future commit, it can break folio_expected_ref_count(), since the folio has private data without elevated refcount. Change them to use folio_attach/detach_private(). Folios during this process are locked as they are in the process of readahead, so no parallel migration/folio split can happen. Furthermore, because folio->private is used to store in-flight I/O counter and the counter reaches 0 when all I/O completes successfully without error or being dirty, ->private=0 causes folio_detach_private() to not drop the elevated folio refcount. Solve this issue by using bias=1 for the counter, so that ->private stays non NULL throughout every attach-to-detach process. Add a macro EROFS_ONLINEFOLIO_BIAS=1. While at it, fix the comment about ->private bit layout and add EROFS_ONLINEFOLIO_COUNT_MASK. It prepares for a future commit that removes PG_private. Assisted-by: LLM To: Gao Xiang <xiang@kernel.org> To: Chao Yu <chao@kernel.org> Cc: Yue Hu <zbestahu@gmail.com> Cc: Jeffle Xu <jefflexu@linux.alibaba.com> Cc: Sandeep Dhavale <dhavale@google.com> Cc: Hongbo Li <hongbohbli@tencent.com> Cc: Chunhai Guo <guochunhai@vivo.com> Cc: linux-erofs@lists.ozlabs.org Cc: linux-kernel@vger.kernel.org Reviewed-by: Gao Xiang <xiang@kernel.org> Signed-off-by: Zi Yan <ziy@nvidia.com> --- fs/erofs/data.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-)
diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index 0885b1f2fc925..f71c7b51bb720 100644
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c@@ -239,19 +239,23 @@ int erofs_map_dev(struct super_block *sb, struct erofs_map_dev *map) /* * bit 30: I/O error occurred on this folio * bit 29: CPU has dirty data in D-cache (needs aliasing handling); - * bit 0 - 29: remaining parts to complete this folio + * bit 0 - 28: remaining parts to complete this folio, biased by 1 so that + * ->private stays non-NULL while the folio is attached */ #define EROFS_ONLINEFOLIO_EIO 30 #define EROFS_ONLINEFOLIO_DIRTY 29 +#define EROFS_ONLINEFOLIO_COUNT_MASK (BIT(EROFS_ONLINEFOLIO_DIRTY) - 1) +#define EROFS_ONLINEFOLIO_BIAS 1 void erofs_onlinefolio_init(struct folio *folio) { union { atomic_t o; void *v; - } u = { .o = ATOMIC_INIT(1) }; + } u = { .o = ATOMIC_INIT(1 + EROFS_ONLINEFOLIO_BIAS) }; - folio->private = u.v; /* valid only if file-backed folio is locked */ + /* valid only if file-backed folio is locked */ + folio_attach_private(folio, u.v); } void erofs_onlinefolio_split(struct folio *folio)
@@ -265,14 +269,14 @@ void erofs_onlinefolio_end(struct folio *folio, int err, bool dirty) do { orig = atomic_read((atomic_t *)&folio->private); - DBG_BUGON(orig <= 0); + DBG_BUGON((orig & EROFS_ONLINEFOLIO_COUNT_MASK) <= EROFS_ONLINEFOLIO_BIAS); v = dirty << EROFS_ONLINEFOLIO_DIRTY; v |= (orig - 1) | (!!err << EROFS_ONLINEFOLIO_EIO); } while (atomic_cmpxchg((atomic_t *)&folio->private, orig, v) != orig); - if (v & (BIT(EROFS_ONLINEFOLIO_DIRTY) - 1)) + if ((v & EROFS_ONLINEFOLIO_COUNT_MASK) != EROFS_ONLINEFOLIO_BIAS) return; - folio->private = 0; + folio_detach_private(folio); if (v & BIT(EROFS_ONLINEFOLIO_DIRTY)) flush_dcache_folio(folio); folio_end_read(folio, !(v & BIT(EROFS_ONLINEFOLIO_EIO)));
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2.53.0