mmb_sync(9)

💾 mmb_sync(9) — the kernel API manual, bestie edition 💾

okay so you found the mmb_sync page in the Kernel API Manual and you're like "what is this even" — don't worry, I got you. 💖

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🌸 NAME 🌸

mmb_sync — write out & wait upon all buffers in a list

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✨ SYNOPSIS ✨

int mmb_sync (struct mapping_metadata_bhs *mmb );

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🎀 ARGUMENTS 🎀

• mmb — the list of buffers to write. that's the whole vibe check, she's just a list of buffers 💅

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💫 DESCRIPTION 💫

okay so first thing: it starts I/O against the buffers in the given list and then waits upon that I/O. literally starts it, then sits there waiting. no cap. 🫶

basically, this is a convenience function for fsync. and mmb is for a file or directory which needs those buffers to be written for a successful fsync, iykyk.

now here's where it gets real — we have conflicting pressures: 💀

• we want to make sure that all initially dirty buffers get waited on
• but that any subsequently dirtied buffers don't

after all, we don't want fsync to last forever if somebody is actively writing to the file. imagine. 😭

so we do this in two main stages:

1. first we copy dirty buffers to a temporary inode list, queueing the writes as we go.
2. then we clean up, waiting for those writes to complete.

and here's the iconic safety detail: mark_buffer_dirty_inode doesn't touch b_assoc_buffers list if b_mmb is not NULL so we are sure the buffer stays on our list until I/O completes (at which point it can be reaped). slay. 🔥

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🦋 SEE ALSO 🦋

kernel file ./fs/buffer.c — and honestly, the whole friend group: bio_endio_bh(9), end_buffer_read_sync(9), bh_end_read(9), bh_end_write(9), bh_end_async_write(9), block_dirty_folio(9), mark_buffer_dirty(9), __brelse(9), __bforget(9), bh_submit(9), bdev_getblk(9), __bread_gfp(9), block_invalidate_folio(9), clean_bdev_aliases(9), try_to_free_buffers(9), bh_uptodate_or_lock(9), __bh_read(9), __bh_read_batch(9)

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ngl bestie, that's a lot of friends but they all ate. go read them too, periodt. 👑

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🌸 okay that's the whole page!! this one came from ./fs and it's dated October 2026, and it's the mmb_sync(9) page from the Kernel API Manual. mwah, go sync something, ily 💖🎀🦋