fault_in_subpage_writeable(9)

💅 fault_in_subpage_writeable(9) — Kernel API Manual, but make it make sense

okay so bestie, this is the fault_in_subpage_writeable(9) page from the Kernel API Manual, and it's lowkey about faulting in an address range for writing. Let's get into it ✨

✨ NAME ✨

fault_in_subpage_writeable — fault in an address range for writing

🌸 SYNOPSIS 🌸

size_t fault_in_subpage_writeable (char __user *uaddr , size_t size );

🎀 ARGUMENTS 🎀

• uaddr — start of address range
• size — size of address range

🦋 DESCRIPTION 🦋

So this function faults in a user address range for writing, and it checks permissions at sub-page granularity (e.g. arm64 MTE, bestie). Use it when the caller cannot guarantee forward progress of a copy_to_user loop — that's the whole reason it exists.

It returns the number of bytes not faulted in (like copy_to_user and copy_from_user). No cap, that's the return value.

💜 SEE ALSO 💜

Kernel file ./mm/gup.c has the whole squad, and here's every single one:

• try_grab_folio(9)
• unpin_user_page(9)
• unpin_folio(9)
• folio_add_pin(9)
• unpin_user_pages_dirty_lock(9)
• unpin_user_page_range_dirty_lock(9)
• unpin_user_pages(9)
• unpin_user_folio(9)
• unpin_folios(9)
• try_grab_folio_fast(9)
• follow_page_mask(9)
• __get_user_pages(9)
• fixup_user_fault(9)
• populate_vma_page_range(9)
• fault_in_writeable(9)
• fault_in_readable(9)
• get_dump_page(9)
• get_user_pages_remote(9)
• get_user_pages(9)
• get_user_pages_fast_only(9)
• get_user_pages_fast(9)
• pin_user_pages_fast(9)
• pin_user_pages_remote(9)
• pin_user_pages(9)
• memfd_pin_folios(9)
• folio_add_pins(9)

okay byeee bestie 🫶 this page comes from the ./mm source, Kernel API Manual section 9, dated October 2026 — periodt 💅