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um: Optimize Flush TLB for force/fork case 

When UML handles a fork the page tables need to be brought up
to date. That was done using brute force - full tlb flush.

This is actually unnecessary, because the mapped-in mappings are
all correct and the only mappings which need to be updated
after a flush are any unmaps (so that paging works) as well as
any pending protection changes.

This optimization squeezes out up to 3% from a full kernel rebuild
time under memory pressure.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
This commit is contained in:
Anton Ivanov 2018-12-07 09:05:53 +00:00 committed by Richard Weinberger
parent 8892d8545f
commit 742f3c8193
1 changed files with 5 additions and 4 deletions
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9
arch/um/kernel/tlb.c
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@ -242,10 +242,11 @@ static inline int update_pte_range(pmd_t *pmd, unsigned long addr,
prot = ((r ? UM_PROT_READ : 0) | (w ? UM_PROT_WRITE : 0) |
(x ? UM_PROT_EXEC : 0));
if (hvc->force || pte_newpage(*pte)) {
if (pte_present(*pte))
ret = add_mmap(addr, pte_val(*pte) & PAGE_MASK,
PAGE_SIZE, prot, hvc);
else
if (pte_present(*pte)) {
if (pte_newpage(*pte))
ret = add_mmap(addr, pte_val(*pte) & PAGE_MASK,
PAGE_SIZE, prot, hvc);
} else
ret = add_munmap(addr, PAGE_SIZE, hvc);
} else if (pte_newprot(*pte))
ret = add_mprotect(addr, PAGE_SIZE, prot, hvc);