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SubjectRe: x86 ptep_get_and_clear question
Jamie Lokier wrote:
> /* mprotect.c */
> entry = ptep_get_and_clear(pte);
> set_pte(pte, pte_modify(entry, newprot));
> I.e. the only code with the race condition is code which explicitly
> clears the dirty bit, in vmscan.c.
> Do you see any possibility of losing a dirty bit here?
Of course.
Just check the output after preprocessing.
int entry;
entry = *pte;
entry &= ~_PAGE_CHG_MASK;
entry |= pgprot_val(newprot)
*pte = entry;

We need
atomic_clear_mask (_PAGE_CHG_MASK, pte);
atomic_set_mask (pgprot_val(newprot), *pte);

for multi threaded apps.

> If not, there's no need for the intricate "gather" or "double scan"
> schemes for mprotect() and it can stay as fast as possible.
Correct, but we need a platform specific "update_pte", and perhaps
update_begin, update_end hooks (empty on i386) for other archs.

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