__anon_vma_prepare(9)

💖 __anon_vma_prepare 💖

okay so like, this is a Kernel API Manual page for __anon_vma_prepare, and it is genuinely so important, bestie, please stay with me 🫶

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✨ NAME ✨

__anon_vma_prepare — attach an anon_vma to a memory region 💅

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🌸 SYNOPSIS 🌸

int __anon_vma_prepare (struct vm_area_struct *vma );

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🎀 ARGUMENTS 🎀

• vma — the memory region in question. that's the one we're talking about 👑

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🦋 DESCRIPTION 🦋

So the whole point of this function: it makes sure that the memory mapping described by vma has an anon_vma attached to it, so that we can associate the anonymous pages mapped into it with that anon_vma. like, it's giving matchmaker energy, fr fr 💖

The common case will be that we already have one — and that case is handled inline by anon_vma_prepare, no drama, no fuss 💅. But if not, we either need to find an adjacent mapping that we can re-use the anon_vma from, or we allocate a new one. And bestie, the adjacent-mapping thing is very common when the only reason for splitting a vma has been mprotect — ikykw 🔥

Now here's where it gets spicy. Anon-vma allocations are very subtle, because we may have optimistically looked up an anon_vma in folio_lock_anon_vma_read and that may actually touch the rwsem even in the newly allocated vma — it depends on RCU to make sure that the anon_vma isn't actually destroyed 😭

As a result (and this is the important part, don't skip it), we need to do proper anon_vma locking even for the new allocation. At the same time, we do not want to do any locking for the common case of already having an anon_vma — because who has the time, honestly ⭐

ngl this one is lowkey delicate — locking is doing real work here, so don't go "optimizing" it away 🙈

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🌈 SEE ALSO 🌈

Kernel file ./mm/rmap.c and its whole friend group 👑 (yes, all of them, every single one, no cap):

• anon_vma_clone(9)
• unlink_anon_vmas(9)
• page_address_in_vma(9)
• folio_referenced(9)
• mapping_wrprotect_range(9)
• pfn_mkclean_range(9)
• folio_move_anon_rmap(9)
• __folio_set_anon(9)
• __page_check_anon_rmap(9)
• folio_add_anon_rmap_ptes(9)
• folio_add_anon_rmap_pmd(9)
• folio_add_new_anon_rmap(9)
• folio_add_file_rmap_ptes(9)
• folio_add_file_rmap_pmd(9)
• folio_add_file_rmap_pud(9)
• folio_remove_rmap_ptes(9)
• folio_remove_rmap_pmd(9)
• folio_remove_rmap_pud(9)
• try_to_unmap(9)
• try_to_migrate(9)
• make_device_exclusive(9)
• __rmap_walk_file(9)

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and that's the whole page, bestie 🧚💜 this one lives in the ./mm source tree and it's dated October 2026 — go forth and attach those anon_vmas, periodt 💅🎀