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SubjectRe: x86 ptep_get_and_clear question
Jamie Lokier wrote:
> Linus Torvalds wrote:
> > So the only case that ends up being fairly heavy may be a case that is
> > very uncommon in practice (only for unmapping shared mappings in
> > threaded programs or the lazy TLB case).
The lazy tlb case is quite fast: lazy tlb thread never write to user
space pages, we don't need to protect the dirty bits. And the first ipi
clears mm->cpu_vm_mask, only one ipi.
> I can think of one case where performance is considered quite important:
> mprotect() is used by several garbage collectors, including threaded
> ones. Maybe mprotect() isn't the best primitive for those anyway, but
> it's what they have to work with atm.

Does mprotect() actually care for wrong dirty bits?
The race should be invisible to user space apps.

>>>>>>> mprotect()
for_all_affected_ptes() {
lock andl ~PERMISSION_MASK, *pte;
lock orl new_permission, *pte;
< now anther cpu could still write to the write protected pages
< and set the dirty bit, but who cares? Shouldn't be a problem.
< tlb flush before ending the syscall, user space can't notice
< the delay.

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