blk_cgroup_mergeable(9)

💅 blk_cgroup_mergeable(9) — the kernel API manual, but make it make sense 🎀

okay so bestie, you know how the kernel has to decide whether two IOs are allowed to get merged together? yeah. this is literally that function. it lives in the <pink>kernel API manual</pink> (that's the manual name, iykyk 🫶) and it's a little bool-returning queen with big main character energy.

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✨ NAME ✨

blk_cgroup_mergeable — determine whether to allow or disallow merges 💅

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🌸 SYNOPSIS 🌸

bool blk_cgroup_mergeable (struct request *rq , struct bio *bio );

that's it. that's the whole signature. two pointers and a bool at the end, periodt. ⭐

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🦋 ARGUMENTS 🦋

• rq — the request you wanna merge into. like, the destination. the host. the main character of the merge 🎀
• bio — the bio you wanna merge. the little incoming guest 💖

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🧚 DESCRIPTION 🧚

so here's the vibe check: bio and rq should belong to the same cgroup — like, same friend group or it's not happening, ngl. and their issue_as_root should match too, fr fr. 🔥

why tho? because — and this is the important part 👑 — we don't want to throttle e.g. a metadata update just because it happens to be sitting right next to a regular IO. imagine getting held back for the crime of standing near someone 😭 not on our watch.

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👀 SEE ALSO 👀

the kernel file ./block/blk-cgroup.h is the spot, and these besties are all in the group chat too 💜:

• bio_issue_as_root_blkg(9)
• blkg_lookup(9)
• blkg_to_pd(9)
• pd_to_blkg(9)
• blkg_get(9)
• blkg_tryget(9)
• blkg_put(9)
• blkg_for_each_descendant_pre(9)
• blkg_for_each_descendant_post(9)
• blkcg_set_delay(9)
• blkcg_clear_delay(9)

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okay and finally, the cute little sign-off 🫶 — this page is from ./block, dated October 2026, and it's filed under blk_cgroup_mergeable(9) in the kernel API manual. slay, go merge responsibly, mwah 💋✨