bio_associate_blkg_from_css(9)

💖 bio_associate_blkg_from_css 💖 (that's the 9 kernel API manual, bestie, we're in the kernel section)

Ok so picture this: you've got a bio (a block I/O request, the main character) and you've got a css (a cgroup subsystem state, its emotional support), and you need them to be, like, linked. This is the function that does the linking. Periodt.

---

✨ NAME ✨

bio_associate_blkg_from_css — associate a bio with a specified css 🫶

---

🌸 SYNOPSIS 🌸

void bio_associate_blkg_from_css (struct bio *bio , struct
       cgroup_subsys_state *css );

No return value, it just does the thing. Void. Iconic behavior honestly 💅

---

🎀 ARGUMENTS 🎀

• bio — the target bio 🎯 like, this is the one getting associated
• css — the target css 🎯 the thing it's getting associated with

---

🦋 DESCRIPTION 🦋

Okay so here's the vibe: this function associates bio with the blkg it finds by combining the css's blkg and the request_queue of the bio. Two ingredients, one blkg, slay 🧚

Now — and this is the important part, don't skip it — if the association fails, that's handled by walking up the blkg tree 🌳. Which means the blkg you end up associated with can be, like, anything between blkg and q->root_blkg. Not a bug, that's just how it goes.

When does that even happen? Only when a cgroup is dying 💀 and then the remaining bios will spill over to the closest alive blkg. Rip to the dying cgroup but those bios are built different, they find a way 🌈

Also! A reference will be taken on the blkg, and it'll be released when bio is freed 🔥 So like, the bookkeeping is handled, you don't gotta worry about it. No cap.

---

👑 SEE ALSO 👑

Kernel file ./block/blk-cgroup.c — go read it, it's giving source of truth 💜

• blkcg_css(9)
• blkg_free(9)
• bio_blkcg_css(9)
• blkcg_parent(9)
• blkg_alloc(9)
• blkg_lookup_create(9)
• blkcg_print_blkgs(9)
• __blkg_prfill_u64(9)
• blkg_conf_init(9)
• blkg_conf_open_bdev(9)
• blkg_conf_prep(9)
• blkg_conf_unprep(9)
• blkg_conf_close_bdev(9)
• blkcg_destroy_blkgs(9)
• blkcg_pin_online(9)
• blkcg_unpin_online(9)
• blkcg_css_offline(9)
• blkcg_activate_policy(9)
• blkcg_deactivate_policy(9)
• blkcg_policy_register(9)
• blkcg_policy_unregister(9)
• blkcg_maybe_throttle_current(9)
• blkcg_schedule_throttle(9)
• blkcg_add_delay(9)
• blkg_tryget_closest(9)
• bio_associate_blkg(9)
• bio_clone_blkg_association(9)

---

💡 friendly reminder bestie: every single one of those above is a real page, go click around, iykyk

---

And that's the whole page! Source <pink>./block</pink>, dated <purple>October 2026</purple>, and yes the whole thing is filed under <hotpink>bio_associate_blkg_from_css(9)</hotpink> 💖 Go be a kernel girlie, you got this 🫶✨🎀