compat_vma_mmap(9)

💅 compat_vma_mmap(9) — Kernel API Manual ✨

Okay so bestie, buckle up, because we're talking about compat_vma_mmap, a little compatibility shim living its best life in the kernel API manual. No cap, it's doing important bridge work while everybody else migrates. 🫶

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🌸 NAME 🌸

compat_vma_mmap — apply the file's .mmap_prepare() hook to an existing VMA and execute any requested actions. That's it, that's the whole vibe. 🎀

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✨ SYNOPSIS ✨

int compat_vma_mmap (struct file *file , struct vm_area_struct *vma );

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🦋 ARGUMENTS 🦋

• file — the file which possesss an f_op->mmap_prepare hook. (And yes, "possesss" has three s's — that's literally how it's written in the source, I'm not fixing it, it's iconic now. 💀)
• vma — the VMA you're applying the .mmap_prepare hook to. That's the main character right there. 👑

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💖 DESCRIPTION 💖

So normally? .mmap_prepare gets invoked directly upon mmap. Simple. Clean. Beautiful. 🌈

But. Certain stacked drivers invoke a nested mmap hook of an underlying file. And that's where things get spicy. 🔥

Until all drivers are converted to use .mmap_prepare, we have to be conservative — we literally must keep invoking these stacked drivers using the deprecated .mmap hook. Lowkey tragic, but that's the situation.

However — and this is the plot twist — we have a problem if the underlying file system possesses an .mmap_prepare hook, because we're in a different context when we invoke the .mmap hook, already having a VMA to deal with. 😭 Different context, different rules, same VMA.

So compat_vma_mmap is the compatibility function that fixes all of this. What it does:

1. Takes VMA state 🫶
2. Establishes a struct vm_area_desc descriptor 📝
3. Passes it to the underlying .mmap_prepare hook 🎯
4. Applies any changes performed by it ✅

Heads up bestie: once the conversion of drivers is complete, this function will no longer be required and will be removed. So don't get too attached. 💔 It's giving limited-edition.

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⭐ RETURN ⭐

0 on success, or error. That's the whole story. Slay. 💅

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🎀 SEE ALSO 🎀

Kernel file ./mm/util.c, plus the whole friend group:

• kfree_const(9)
• __kmemdup_nul(9)
• kstrdup(9)
• kstrdup_const(9)
• kstrndup(9)
• kmemdup(9)
• kmemdup_array(9)
• kvmemdup(9)
• kmemdup_nul(9)
• memdup_user(9)
• vmemdup_user(9)
• strndup_user(9)
• memdup_user_nul(9)
• randomize_page(9)
• __account_locked_vm(9)
• account_locked_vm(9)
• __vmalloc_array(9)
• vmalloc_array(9)
• __vcalloc(9)
• vcalloc(9)
• folio_mapping(9)
• folio_copy(9)
• get_cmdline(9)
• mem_dump_obj(9)
• compat_set_desc_from_vma(9)
• __compat_vma_mmap(9)
• snapshot_page(9)
• mmap_action_prepare(9)
• mmap_action_complete(9)
• folio_pte_batch(9)
• page_range_contiguous(9)

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And that's the page!! 💜 Big kiss from the ./mm directory, dated October 2026 — go forth and call compat_vma_mmap, bestie, while it still exists. 🧚🍓🌸