Why do we set _PAGE_DIRTY for page tables?

I just noticed this:

#define _PAGE_TABLE (_PAGE_PRESENT | _PAGE_RW | _PAGE_USER | \
_PAGE_ACCESSED | _PAGE_DIRTY)
#define _KERNPG_TABLE (_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED | \
_PAGE_DIRTY)

Is there a reason we set _PAGE_DIRTY for page tables? It has no function, but doesn't do any harm either (the dirty bit is ignored for page tables)... it just looks funny to me.

-hpa

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I think it just got copied, and at least the A bit does matter even in page tables (well, it gets updated, I don't know how much that "matters"). So the fact that D is ignored is actually the odd man out.

Linus

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Yes, not setting the A bit means the hardware will take an assist to set the bit for us, which is a waste of time if we don't care about it. The D bit is the one which made me wonder; I thought either it was just copy & paste, or that it got set to make it more analogous with large pages.

-hpa

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