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[131.94.134.89]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-5c3d05c594fsm3706321e0c.8.2026.08.06.09.59.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 06 Aug 2026 09:59:07 -0700 (PDT) From: Chao Shi To: Jan Kara , Christian Brauner , Alexander Viro , Matthew Wilcox , linux-fsdevel@vger.kernel.org Cc: Theodore Ts'o , Andreas Dilger , Baokun Li , Ojaswin Mujoo , Ritesh Harjani , Zhang Yi , Zhang Yi , Bob Copeland , Namjae Jeon , Sungjong Seo , Yuezhang Mo , OGAWA Hirofumi , Mark Fasheh , Joel Becker , Joseph Qi , Andreas Gruenbacher , linux-ext4@vger.kernel.org, ocfs2-devel@lists.linux.dev, gfs2@lists.linux.dev, linux-kernel@vger.kernel.org, Chao Shi , Weidong Zhu Subject: [PATCH v2 03/21] jbd2: point the shadow buffer at the frozen data directly Date: Thu, 6 Aug 2026 12:58:26 -0400 Message-ID: <6140cd23beb88e99f40eaeff4044a16213f6caab.1785951556.git.coshi036@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit When a metadata buffer has to be copied out before it can be journalled, jbd2_journal_write_metadata_buffer() writes jh->b_frozen_data rather than the page cache copy. b_frozen_data is kmalloc()ed, so folio_set_bh() makes the shadow buffer point at a slab folio. That is not something the buffer_head layer can reason about. A slab folio overloads ->mapping, so a shadow buffer looks like it belongs to an address_space when it does not. buffer_set_crypto_ctx() already has to work around this, and it is the reason mark_buffer_write_io_error() cannot be called on a shadow buffer today. Point the shadow buffer at the frozen data itself instead: leave b_folio NULL, which it already is out of alloc_buffer_head(), and set b_data. The previous patch taught fs/buffer.c to submit such a buffer. folio_set_bh() is now needed on only one path - the one that journals the page cache copy directly - so it moves there, and new_folio, new_offset and the flag that used to pick between them all go away. The two commit-path checksum helpers reach the shadow buffer's contents through a new kmap_local_bh()/kunmap_local_bh() pair, which handle a buffer with or without a folio. Memory outside the page cache is always mapped, so for those there is nothing to map or unmap. Mapping it anyway would be worse than pointless: with CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP, kmap_local_page() hands back a one page mapping even for such memory, which is not enough for a buffer bigger than a page. Tested with ext4 mounted data=journal,journal_checksum on a metadata_csum filesystem, writing files whose every block begins with the JBD2 magic so that escaping forces the copy-out, then crashing with sysrq-b without unmounting and replaying the journal on the next mount. Recovery completed, the file contents matched, e2fsck -fn was clean, and an instrumented build confirmed the b_folio == NULL path was taken. Suggested-by: Matthew Wilcox (Oracle) Acked-by: Weidong Zhu Signed-off-by: Chao Shi --- fs/jbd2/commit.c | 8 ++++---- fs/jbd2/journal.c | 29 +++++++++++++++++------------ include/linux/buffer_head.h | 29 +++++++++++++++++++++++++++++ 3 files changed, 50 insertions(+), 16 deletions(-) diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c index 3029cb6f6d64..0c85af91f9b2 100644 --- a/fs/jbd2/commit.c +++ b/fs/jbd2/commit.c @@ -330,9 +330,9 @@ static __u32 jbd2_checksum_data(__u32 crc32_sum, struct buffer_head *bh) char *addr; __u32 checksum; - addr = kmap_local_folio(bh->b_folio, bh_offset(bh)); + addr = kmap_local_bh(bh); checksum = crc32_be(crc32_sum, addr, bh->b_size); - kunmap_local(addr); + kunmap_local_bh(bh, addr); return checksum; } @@ -357,10 +357,10 @@ static void jbd2_block_tag_csum_set(journal_t *j, journal_block_tag_t *tag, return; seq = cpu_to_be32(sequence); - addr = kmap_local_folio(bh->b_folio, bh_offset(bh)); + addr = kmap_local_bh(bh); csum32 = jbd2_chksum(j->j_csum_seed, (__u8 *)&seq, sizeof(seq)); csum32 = jbd2_chksum(csum32, addr, bh->b_size); - kunmap_local(addr); + kunmap_local_bh(bh, addr); if (jbd2_has_feature_csum3(j)) tag3->t_checksum = cpu_to_be32(csum32); diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c index 09efa337649e..6e05dc47e20a 100644 --- a/fs/jbd2/journal.c +++ b/fs/jbd2/journal.c @@ -327,8 +327,6 @@ int jbd2_journal_write_metadata_buffer(transaction_t *transaction, { int do_escape = 0; struct buffer_head *new_bh; - struct folio *new_folio; - unsigned int new_offset; struct buffer_head *bh_in = jh2bh(jh_in); journal_t *journal = transaction->t_journal; @@ -348,24 +346,31 @@ int jbd2_journal_write_metadata_buffer(transaction_t *transaction, /* keep subsequent assertions sane */ atomic_set(&new_bh->b_count, 1); + /* + * b_frozen_data is slab memory, not page cache, so when we use it the + * shadow buffer gets no folio at all: b_folio stays NULL from the + * allocation and b_data points straight at the copy. Pointing it at + * the slab folio instead would hand its overloaded ->mapping to + * anything that goes looking for an address_space. + */ + spin_lock(&jh_in->b_state_lock); /* * If a new transaction has already done a buffer copy-out, then * we use that version of the data for the commit. */ if (jh_in->b_frozen_data) { - new_folio = virt_to_folio(jh_in->b_frozen_data); - new_offset = offset_in_folio(new_folio, jh_in->b_frozen_data); do_escape = jbd2_data_needs_escaping(jh_in->b_frozen_data); if (do_escape) jbd2_data_do_escape(jh_in->b_frozen_data); + new_bh->b_data = jh_in->b_frozen_data; } else { + struct folio *folio = bh_in->b_folio; + unsigned int offset = offset_in_folio(folio, bh_in->b_data); char *tmp; char *mapped_data; - new_folio = bh_in->b_folio; - new_offset = offset_in_folio(new_folio, bh_in->b_data); - mapped_data = kmap_local_folio(new_folio, new_offset); + mapped_data = kmap_local_folio(folio, offset); /* * Fire data frozen trigger if data already wasn't frozen. Do * this before checking for escaping, as the trigger may modify @@ -379,8 +384,10 @@ int jbd2_journal_write_metadata_buffer(transaction_t *transaction, /* * Do we need to do a data copy? */ - if (!do_escape) + if (!do_escape) { + folio_set_bh(new_bh, folio, offset); goto escape_done; + } spin_unlock(&jh_in->b_state_lock); tmp = kmalloc(bh_in->b_size, GFP_NOFS | __GFP_NOFAIL); @@ -391,7 +398,7 @@ int jbd2_journal_write_metadata_buffer(transaction_t *transaction, } jh_in->b_frozen_data = tmp; - memcpy_from_folio(tmp, new_folio, new_offset, bh_in->b_size); + memcpy_from_folio(tmp, folio, offset, bh_in->b_size); /* * This isn't strictly necessary, as we're using frozen * data for the escaping, but it keeps consistency with @@ -400,13 +407,11 @@ int jbd2_journal_write_metadata_buffer(transaction_t *transaction, jh_in->b_frozen_triggers = jh_in->b_triggers; copy_done: - new_folio = virt_to_folio(jh_in->b_frozen_data); - new_offset = offset_in_folio(new_folio, jh_in->b_frozen_data); jbd2_data_do_escape(jh_in->b_frozen_data); + new_bh->b_data = jh_in->b_frozen_data; } escape_done: - folio_set_bh(new_bh, new_folio, new_offset); new_bh->b_size = bh_in->b_size; new_bh->b_bdev = journal->j_dev; new_bh->b_blocknr = blocknr; diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h index 699970b4bbf2..20b8fca1abfa 100644 --- a/include/linux/buffer_head.h +++ b/include/linux/buffer_head.h @@ -172,6 +172,35 @@ static inline unsigned long bh_offset(const struct buffer_head *bh) return (unsigned long)(bh)->b_data & (folio_size(bh->b_folio) - 1); } +/** + * kmap_local_bh - Map the data of a buffer. + * @bh: The buffer. + * + * Buffers usually live in the page cache, but a few are built over memory + * which is not. Those carry no folio and b_data is already a kernel address + * which is always mapped, so there is nothing to do for them. Pair with + * kunmap_local_bh(). + * + * Return: A pointer to the buffer's data. + */ +static inline void *kmap_local_bh(const struct buffer_head *bh) +{ + if (!bh->b_folio) + return bh->b_data; + return kmap_local_folio(bh->b_folio, bh_offset(bh)); +} + +/** + * kunmap_local_bh - Unmap the data of a buffer. + * @bh: The buffer. + * @addr: The address returned by kmap_local_bh(). + */ +static inline void kunmap_local_bh(const struct buffer_head *bh, void *addr) +{ + if (bh->b_folio) + kunmap_local(addr); +} + /* If we *know* page->private refers to buffer_heads */ #define page_buffers(page) \ ({ \ -- 2.43.0