VM_PAGE_BITS(9)

💫 vm_page_bits 💫

okay so bestie, this is the VM_PAGE_BITS(9) manual page — Kernel Developer's Manual energy fr fr, and it's literally all about managing page clean and dirty bits ✨ it's giving low-level filesystem realness, and i'm obsessed 🫶

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💖 NAME 💖

vm_page_bits, vm_page_set_validclean, vm_page_clear_dirty, vm_page_set_invalid, vm_page_zero_invalid, vm_page_is_valid, vm_page_test_dirty, vm_page_dirty, vm_page_undirty — manage page clean and dirty bits, periodt 💅

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✨ SYNOPSIS ✨

#include <sys/param.h>
#include <vm/vm.h>
#include <vm/vm_page.h>

int
vm_page_bits(int base, int size);

void
vm_page_set_validclean(vm_page_t m, int base, int size);

void
vm_page_clear_dirty(vm_page_t m, int base, int size);

void
vm_page_set_invalid(vm_page_t m, int base, int size);

void
vm_page_zero_invalid(vm_page_t m, boolean_t setvalid);

int
vm_page_is_valid(vm_page_t m, int base, int size);

void
vm_page_test_dirty(vm_page_t m);

void
vm_page_dirty(vm_page_t m);

void
vm_page_undirty(vm_page_t m);

like, these are the includes you need first 👆 no cap, don't skip them 🙈

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🦋 DESCRIPTION 🦋

okay so let's break these down one by one, bestie — each function is iconic in its own way 🌸

• vm_page_bits() — calculates the bits representing the DEV_BSIZE range of bytes between base and size. the byte range is supposed to be within a single page, and if size is zero, no bits get set. that's it, that's the function 💖
• vm_page_set_validclean() — flags the byte range between base and size as <green>valid</green> and <mint>clean</mint>. the range is expected to be DEV_BSIZE aligned and no larger than PAGE_SIZE. heads up tho 💀 if it's not properly aligned, any unaligned chunks of the DEV_BSIZE blocks at the beginning and end of the range get zeroed.

alsooo — if base is zero and size is one page, the modified bit in the page map is cleared, and the VPO_NOSYNC flag is cleared too. two for one, slay 🎀

• vm_page_clear_dirty() — clears the dirty bits within a page in the range between base and size. the bits representing the range are calculated by calling vm_page_bits(). so it's giving teamwork, love that for them 🫶
• vm_page_set_invalid() — clears the bits in both the valid and dirty flags representing the DEV_BSIZE blocks between base and size in the page. the bits are calculated by calling vm_page_bits(). and as well as clearing the bits within the page, the generation number within the object holding the page gets incremented. main character energy, fr fr 👑
• vm_page_zero_invalid() — zeroes all of the blocks within the page that are currently flagged as invalid. if setvalid is TRUE, all of the valid bits within the page are set. easy peasy ✨

ngl in some cases — like NFS — the valid bits can't be set, because cache consistency has to be maintained 🧚 so like, it's not always a free-for-all, iykyk.

• vm_page_is_valid() — checks to determine if all of the DEV_BSIZE blocks between base and size of the page are valid. lowkey a little quirky with the edge case: if size is zero and the page is entirely invalid, vm_page_is_valid() will return TRUE. in all other cases, a size of zero will return FALSE. so watch out for that 💀
• vm_page_test_dirty() — checks if a page has been modified via any of its physical maps, and if so, flags the entire page as dirty. vm_page_dirty() is called to modify the dirty bits. detective work but make it kernel 🌈
• vm_page_dirty() — flags the entire page as dirty. it's expected that the page is not currently on the cache queue. like, don't do that, okay? 🙈
• vm_page_undirty() — clears all of the dirty bits in a page. clean slate, fresh start, we love a reset 🦋

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💜 NOTES 💜

none of these functions are allowed to block. like, at all. blocking is not the vibe here, periodt 🛑

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🎀 AUTHORS 🎀

this manual page was written by Chad David <davidc@acns.ab.ca>. slay, Chad 💖

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and that's the whole manual, bestie! this page is from the Debian distribution, dated December 1, 2001 🎀 so yeah, it's a little vintage but the facts are still facts 💅 ✨