blk_rq_map_user_iov(9)

💫 blk_rq_map_user_iov — the kernel API manual, but make it make sense

okay so bestie, welcome to the <purple>Kernel API Manual<//purple> entry for blk_rq_map_user_iov(9) — the function that maps your user data onto a request so passthrough requests can go brrr. it's giving "glue code but make it iconic" 💅

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✨ SYNOPSIS ✨

int blk_rq_map_user_iov (struct request_queue *q , struct request *rq ,
struct rq_map_data *map_data , const struct iov_iter *iter , gfp_t gfp_mask );

six arguments, one int coming back at you. yes that's a lot of pointers. no you're not allowed to skip any of them. 🫶

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🎀 ARGUMENTS 🎀

• q — the request queue where the request should be inserted. like, where does this request live? right here, bestie. 🏠
• rq — the request you're mapping data to. the main character, honestly. 👑
• map_data — pointer to the rq_map_data holding the pages (if necessary). note the "if necessary" — it's conditional, iykyk.
• iter — the iovec iterator. this is the actual user data you're feeding in. 🍓
• gfp_mask — memory allocation flags. the boring-but-load-bearing one. ngl you need these.

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🌸 DESCRIPTION 🌸

okay so here's the vibe: data gets mapped directly for zero copy I/O if possible. zero copy!! we love efficiency!! 🔥

but if that's not possible? a kernel bounce buffer is used instead. not the glow-up we wanted, but it's the fallback and it works. 💀

🚨 real talk: you must issue a matching blk_rq_unmap_user at the end of the I/O, and you gotta do it while still in process context. don't just leave it mapped and walk away, that's not the move, periodt. 🙈

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🦋 SEE ALSO 🦋

• kernel file ./block/blk-map.c — where the magic happens
• bio_copy_from_iter(9)
• bio_copy_to_iter(9)
• bio_uncopy_user(9)
• bio_copy_kern(9)
• blk_rq_unmap_user(9) — the one you literally cannot forget (see above ⬆️)
• blk_rq_map_kern(9)

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💖 cute little sign-off 💖

this page lives down in ./block, dated October 2026, brought to you by the <rainbow>Kernel API Manual<//rainbow>. go map those iovecs, bestie — and remember the unmap. mwah 💋✨