UFFDIO_RWPROTECT(2const)

💖 UFFDIO_RWPROTECT 💖

okay bestie, gather round, because we're talking about a 2const man page today — that's right, a constant, not a whole function, and she is serving. This one lives in the Linux userfaultfd cinematic universe and her whole job is: read-write-protect or un-protect a userfaultfd-registered memory range. ✨

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📚 LIBRARY

Standard C library (libc, -lc) — the usual queen.

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🧚 SYNOPSIS

#include <linux/userfaultfd.h>  /* Definition of UFFD* constants */
#include <sys/ioctl.h>

int ioctl(int fd, UFFDIO_RWPROTECT, struct uffdio_rwprotect *argp);

#include <linux/userfaultfd.h>

struct uffdio_rwprotect {
    struct uffdio_range  range;  /* Range to change RWP on */
    __u64                mode;   /* Mode flags */
};

like, two fields. that's it. she's minimal, she's clean, she's iconic. 💅

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🌸 DESCRIPTION

So basically: you use this to read-write-protect (or un-protect) a memory range that you already registered with the mode UFFDIO_REGISTER_MODE_RWP. RWP stands for read-write-protect, iykyk. 🎀

The following mode bits are supported:

• UFFDIO_RWPROTECT_MODE_RWP — when this mode bit is set, the ioctl installs read-write protection on every page present in the range you specified with .range. If it's not set, the ioctl instead removes read-write protection from the range — which is also literally how a fault handler resolves an UFFD_PAGEFAULT_FLAG_RWP notification. no cap. 🔥
• UFFDIO_RWPROTECT_MODE_DONTWAKE — when this mode bit is set, do not wake up any thread that's waiting for page-fault resolution after the operation. careful though bestie — you can only specify this if UFFDIO_RWPROTECT_MODE_RWP is not specified.

Now for the lore that actually matters. Read-write protection only affects pages that are currently populated in the range — unmapped addresses are left untouched, she simply does not perceive them. 💀

For anonymous mappings, protection is preserved across page reclaim (the marker rides on the swap entry, cute) and migration. For shmem and file-backed mappings, protection is dropped when the backing page is reclaimed — yeah, she ghosts you, it's giving tragic. 😭

⚠️ Also: callers must re-arm a range with UFFDIO_RWPROTECT after any operation that explicitly drops the underlying page: - MADV_DONTNEED on anonymous memory - hole-punch on shmem - truncation of a file mapping

don't forget to re-arm, or you will be sad. 🙈

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👑 RETURN VALUE

On success, 0 is returned. On error, -1 is returned and errno is set to indicate the error. classic. periodt. ✨

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💀 ERRORS

• EINVAL — the start or the len field of the uffdio_range structure was not a multiple of the system page size, or len was zero, or the specified range was otherwise invalid, or an invalid mode bit was specified, or UFFDIO_RWPROTECT_MODE_DONTWAKE was specified together with UFFDIO_RWPROTECT_MODE_RWP. yeah that's a lot of ways to be wrong, we don't judge. 🌸
• EAGAIN — the process was interrupted; retry this call. just run it back bestie. 🦋
• ENOENT — the range specified in .range is not valid; for example, the virtual address does not exist, or part of the range is not registered with UFFDIO_REGISTER_MODE_RWP. she's looking for something that isn't there, it's giving searching-for-your-ex's-instagram. 📱
• EFAULT — encountered a generic fault during processing. generic. no notes. 💅

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🌈 STANDARDS

Linux. (that's the whole vibe check, she passes.) ⭐

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🎀 HISTORY

Linux 7.2. baby of the family, freshly minted. 👑

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🍓 EXAMPLES

See userfaultfd(2). we're not doing the whole routine here, the main character page handles it. ✨

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🔥 SEE ALSO

• ioctl(2)
• ioctl_userfaultfd(2)
• userfaultfd(2)
• linux.git/Documentation/admin-guide/mm/userfaultfd.rst

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mwah 💖 that's the whole page. This lil UFFDIO_RWPROTECT(2const) beauty lives in Linux man-pages 6.20, dated 2026-05-28. go protect some pages responsibly, bestie — and remember to re-arm. 🫶🌸💜