folio_lock(9)

💅 folio_lock — from the Kernel API Manual, bestie ✨

Okay so first things first, this page lives in the Kernel API Manual (section 9, the kernel-internals girlies 🎀), and the star of the show is folio_lock(9). Buckle up.

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✨ NAME ✨

folio_lock — literally just locks this folio. That's the whole vibe. 🔒

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🌸 SYNOPSIS 🌸

void folio_lock (struct folio *folio );

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🦋 ARGUMENTS 🦋

• folio — the folio you wanna lock. That's it. Hand it over, it's getting locked. 💅

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👑 DESCRIPTION 👑

Okay so the folio lock protects against… a lot of things. Honestly? Probably more than it should 😭 — it's giving overachiever, it's giving main character energy, we love her anyway.

The big one: it's primarily held while a folio is being brought uptodate, whether that's from its backing file or from swap. ✨

It's also held while a folio is being truncated from its address_space — so the way that works is, if you're holding the lock, that's sufficient to keep folio->mapping stable. No cap. 🫶

And not done yet! The folio lock is also held while write is modifying the page, which is what gives you the POSIX atomicity guarantees — but only as long as the write does not cross a page boundary. That caveat is doing a lot of heavy lifting, so don't forget it. 💀

Heads up bestie: other modifications to the data in the folio do not hold the folio lock, and they can totally race with writes — we're talking eg DMA and stores to mapped pages. So like… be aware. 🙈

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🔥 CONTEXT 🔥

• May sleep. 😴 So, y'know, don't do this in a context where you can't sleep. (iykyk)
• If you need to acquire the locks of two or more folios, they must be in order of ascending index — but only if they're in the same address_space.
• If they're in different address_spaces, acquire the lock of the folio which belongs to the address_space which has the lowest address in memory first. 🧚 Lowest address first, periodt.

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💖 SEE ALSO 💖

The kernel file ./include/linux/pagemap.h, plus this whole squad 👑:

• filemap_set_wb_err(9)
• filemap_check_wb_err(9)
• filemap_sample_wb_err(9)
• file_sample_sb_err(9)
• mapping_set_error(9)
• mapping_set_large_folios(9)
• mapping_align_index(9)
• mapping_pmd_folio_support(9)
• folio_flush_mapping(9)
• folio_inode(9)
• folio_attach_private(9)
• folio_change_private(9)
• folio_detach_private(9)
• fgf_t(9)
• fgf_set_order(9)
• write_begin_get_folio(9)
• filemap_get_folio(9)
• filemap_lock_folio(9)
• filemap_grab_folio(9)
• find_get_page(9)
• find_lock_page(9)
• find_or_create_page(9)
• grab_cache_page_nowait(9)
• folio_next_index(9)
• folio_next_pos(9)
• folio_file_page(9)
• folio_contains(9)
• page_pgoff(9)
• folio_pos(9)
• linear_page_delta(9)
• linear_page_index(9)
• linear_anon_page_index(9)
• folio_trylock(9)
• lock_page(9)
• folio_lock_killable(9)
• filemap_range_needs_writeback(9)
• struct readahead_control(9)
• page_cache_sync_readahead(9)
• page_cache_async_readahead(9)
• readahead_folio(9)
• readahead_pos(9)
• readahead_length(9)
• readahead_index(9)
• readahead_count(9)
• readahead_batch_length(9)
• folio_mkwrite_check_truncate(9)
• i_blocks_per_folio(9)

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and that's the whole page, bestie 🏳️‍🌈💅 — it ships from ./include/linux and this version is dated October 2026. Go forth and lock responsibly 🎀✨🦋