ext4_get_branch(9)

💖 ext4_get_branch — bestie's guide to the kernel API manual ✨

okay so like, this is ext4_get_branch(9) from the Kernel API Manual, and honestly? she's a baddie. let's get into it 🎀

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🌸 NAME 🌸

ext4_get_branch — read the chain of indirect blocks leading to data 🦋

like, the whole vibe is: follow the chain of indirect blocks until you find the actual data. it's giving treasure map energy iykyk 💅

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✨ SYNOPSIS ✨

Indirect * ext4_get_branch (struct inode *inode , int depth ,
       ext4_lblk_t *offsets , Indirect chain[4] , int *err );

no cap, that's the exact signature, don't come for me 😭

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🫶 ARGUMENTS 🫶

• inode — the inode in question, aka the main character 👑
• depth — depth of the chain (1 - direct pointer, etc.) — yes, that's literally how it's written, 1 - direct pointer, etc. 🧚
• offsets — offsets of pointers in inode/indirect blocks
• chain — place to store the result (she's the one holding everything together fr fr)
• err — here we store the error value 💀

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🔥 DESCRIPTION 🔥

so the function fills the array of triples <key, p, bh> and returns NULL if everything went OK, or the pointer to the last filled triple (the incomplete one) otherwise. periodt.

upon the return:

• chain[i].key contains the number of (i+1)-th block in the chain (as it is stored in memory, i.e. little-endian 32-bit) — don't ask me why little-endian, that's just the lore 🦋
• chain[i].p contains the address of that number (it points into struct inode for i==0 and into the bh->b_data for i>0)
• chain[i].bh points to the buffer_head of i-th indirect block for i>0 and NULL for i==0

in other words, it holds the block numbers of the chain, addresses they were taken from (and where we can verify that chain did not change) and buffer_heads hosting these numbers. literally the receipts for everything 💅

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🎀 when does she stop 🎀

• when it stumbles upon zero pointer (absent block) → pointer to last triple returned, *err == 0
• when it gets an IO error reading an indirect block → ditto, *err == -EIO 😭
• when it reads all depth-1 indirect blocks successfully and finds the whole chain, all way to the data → returns NULL, *err == 0 slay ✨

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⚠️ ok bestie listen: need to be called with down_read(EXT4_I(inode)->i_data_sem) — do NOT skip this, it's not optional, it's the vibe check 🌈

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🌟 SEE ALSO 🌟

Kernel file ./fs/ext4/indirect.c:

ext4_block_to_path(9), ext4_find_near(9), ext4_find_goal(9), ext4_blks_to_allocate(9), ext4_alloc_branch(9), ext4_splice_branch(9), ext4_find_shared(9), ext4_free_data(9), ext4_free_branches(9), ext4_ind_remove_space(9)

the whole friend group fr, go read them too 🫶

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and that's the page!! this icon came from ./fs/ext4, dated October 2026, and it's part of the ext4_get_branch(9) kernel API manual — mwah, love u, go read your indirect blocks responsibly 💜🥰🍓