compat_set_desc_from_vma(9)

💅 compat_set_desc_from_vma(9) — from the Kernel API Manual, bestie ✨

Okay so buckle up, because this one is a tiny little function with a big identity crisis (affectionate). It lives in section 9 of the Kernel API Manual, and it's literally called compat_set_desc_from_vma. Let's get into it 🫶

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✨ NAME ✨

compat_set_desc_from_vma — assigns VMA descriptor desc fields from a VMA. That's the whole job. It takes a VMA, and it fills in a descriptor from it. Iconic, efficient, no notes.

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🌸 SYNOPSIS 🌸

void compat_set_desc_from_vma (struct vm_area_desc *desc , const struct
file *file , const struct vm_area_struct *vma );

Note the void — it gives nothing back, it just does the thing and returns. Main character energy honestly 💅

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🎀 ARGUMENTS 🎀

• desc — a VMA descriptor whose fields need to be set. Like, this is the glow-up target. Its fields are currently unset and we're about to fix that ✨
• file — the file object describing the file being mmap'd. Yes, that file. The one you're mapping 💖
• vma — the VMA whose fields we wish to assign to desc. This is the source of truth, the origin story, the blueprint 🌸

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🦋 DESCRIPTION 🦋

So here's the tea: this is a compatibility function. It exists so that an mmap hook can call mmap_prepare hooks when drivers nest these. That's the whole reason it's here — it's a bridge, a little social connector between two worlds 🫶

Specifically (and this is the important bit, don't skip it), it allows the construction of a vm_area_desc value, desc, from a VMA, vma, for the purposes of doing this. So: VMA goes in, descriptor comes out, nesting drivers get to talk to each other. Slay 🔥

⚠️ Real talk: once the conversion of drivers is complete, this function will no longer be required and will be removed. It's on borrowed time, bestie. Don't build your whole personality around it 💀

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🌈 SEE ALSO 🌈

The Kernel file ./mm/util.c, plus this entire extended 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_vma_mmap(9), compat_vma_mmap(9), snapshot_page(9), mmap_action_prepare(9), mmap_action_complete(9), folio_pte_batch(9), page_range_contiguous(9)

Go read them all. Or at least the compat_vma_mmap ones, they're basically siblings 👑

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Mwah 💋 this page hails from ./mm, dated October 2026, and that's the vibe check complete. Go forth and set those descriptor fields, bestie 🧚