memcpy_from_file_folio(9)

📦 version 7.3.1, in nixpkgs unstable

💖 memcpy_from_file_folio — the Kernel API Manual, bestie edition 💖

Okay so like, this one lives in the Kernel API Manual (section 9, the kernel-functions girlies ✨) and honestly? It's giving "small function, big responsibility." Let me put you on 🫶

🌸 NAME 🌸

memcpy_from_file_folio — literally just copy some bytes from a file folio. That's it. That's the whole vibe. 💅

✨ SYNOPSIS ✨

size_t memcpy_from_file_folio (char *to , struct folio *folio , loff_t
pos , size_t len );

Like, look at her. Returns a size_t, takes four args, no drama. 🎀

🦋 ARGUMENTS 🦋

  • to — the destination buffer, aka where the bytes are moving to. She's the receiving end, the main character of the copy operation 💖
  • folio — the folio you're copying from. The source. The origin story ✨
  • pos — the position in the file. Like, where are we even, bestie? 📍
  • len — the maximum number of bytes to copy. Max, not guaranteed — remember that, it comes back later 👀

💜 DESCRIPTION 💜

So the whole point: copy up to len bytes from this folio. Copy up to — not "exactly," not "at least," up to 🔥

And the way the actual amount can get capped? Two things:

  • It may be limited by PAGE_SIZE if the folio comes from HIGHMEM (yes, HIGHMEM is still a thing and she's still out here applying limits 😭)
  • It's also limited by the size of the folio itself, obviously, because you can't copy more bytes than actually exist, no cap 💀

So like, do the vibe check: you asked for len, but the folio might be like "bestie, no, here's less." 🙈

⭐ RETURN ⭐

The number of bytes actually copied from the folio. So check it! Don't just assume you got your full len, because you might not have 😌

🌈 SEE ALSO 🌈

Kernel file ./include/linux/highmem.h — and the whole highmem friend group, honestly, they all show up together 👑:

iykyk 🧚

🎀 Friendly reminder fr fr: len is a maximum, the folio and PAGE_SIZE under HIGHMEM get a vote too, and the return value is your receipts. Periodt. 💅

Source: ./include/linux — dated October 2026, from the Kernel API Manual, section 9. Bye bestie, go copy some bytes responsibly 🫶💖✨

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