__swap_cache_add_check(9)

📦 version 7.3.1, in nixpkgs unstable

💅 __swap_cache_add_check — the kernel API manual, but make it make sense

okay so bestie, we're in __swap_cache_add_check(9) territory today and honestly? it's giving "quiet little helper function that does a LOT of checking." no cap. let's get into it 🫶

✨ NAME ✨

__swap_cache_add_check — literally just vibe-checks whether a range is even allowed to have a folio added to it. that's it. that's the whole job. 🎀

✨ SYNOPSIS ✨

int __swap_cache_add_check (struct swap_cluster_info *ci , swp_entry_t
targ_entry , unsigned long nr , void **shadowp , unsigned short
*memcg_id );

✨ ARGUMENTS ✨

here's the roster, bestie, and every single one of them matters 💖

  • ci — the locked swap cluster. yes, locked. it says locked. do not come to this function with an unlocked cluster, that's not the vibe 😭
  • targ_entry — the target swap entry you want checked. heads up: it gets rounded down by nr. she snaps to the grid, periodt.
  • nr — how many slots we're checking. must be a power of 2. like, non-negotiable. powers of 2 only, that's the rule 🌸
  • shadowp — this one returns the shadow value if one exists in the range. so if there's a shadow lurking in there, you'll get it back here.
  • memcg_id — returns the memory cgroup id. pass NULL if you want to ignore the cgroup check entirely. your call, bestie ✨

✨ DESCRIPTION ✨

okay so here's what she actually does 👑

she checks if all the slots covered by the given range have a swap count >= 1. all of them. every single one. no skipping, no "close enough," none of that 🙈

while she's in there, she also retrieves the shadow if there is one. so that's two birds, one function call, we love efficiency 💅

and if memcg_id is not NULL? she goes the extra mile and also checks whether all the slots belong to the same cgroup, and returns the cgroup private id. icon behavior, honestly 🔥

✨ CONTEXT ✨

⚠️ real talk: the caller must lock the cluster. this is not optional. this is not a suggestion. lock the cluster before you call her or things get weird 💀

✨ RETURN ✨

  • 0 if success — slay, you're good to go 🎉
  • error code if failed — something went wrong, read the code, you got this 😭

✨ SEE ALSO ✨

kernel file ./mm/swap_state.c plus the whole friend group:

go read them too, iykyk 🦋

mwah 💜 that's the whole page, brought to you fresh out of ./mm, dated October 2026, and officially filed under __swap_cache_add_check(9). stay locked, stay checking, stay iconic 👑✨

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