On Mon, Oct 01, 2012 at 03:59:44PM +0100, Andrea Arcangeli wrote:
> Hi Will,
Hi Andrea, Kirill,
Thanks for the comments.
> On Mon, Oct 01, 2012 at 02:51:45PM +0100, Will Deacon wrote:
> > +void huge_pmd_set_accessed(struct mm_struct *mm, struct vm_area_struct *vma,
> > + unsigned long address, pmd_t *pmd, pmd_t orig_pmd)
> > +{
> > + pmd_t entry;
> > +
> > + spin_lock(&mm->page_table_lock);
> > + entry = pmd_mkyoung(orig_pmd);
> > + if (pmdp_set_access_flags(vma, address & HPAGE_PMD_MASK, pmd, entry, 0))
> > + update_mmu_cache(vma, address, pmd);
>
> If the pmd is being splitted, this may not be a trasnhuge pmd anymore
> by the time you obtained the lock. (orig_pmd could be stale, and it
> wasn't verified with pmd_same either)
>
> The lock should be obtained through pmd_trans_huge_lock.
>
> if (pmd_trans_huge_lock(orig_pmd, vma) == 1)
> {
> set young bit
> spin_unlock(&mm->page_table_lock);
> }
I didn't notice that -- thanks. I'll move the locking outside of the
_set_accessed function and direct it via that function instead.
> On x86:
>
> int pmdp_set_access_flags(struct vm_area_struct *vma,
> unsigned long address, pmd_t *pmdp,
> pmd_t entry, int dirty)
> {
> int changed = !pmd_same(*pmdp, entry);
>
> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>
> if (changed && dirty) {
> *pmdp = entry;
>
> with dirty == 0 it looks like it won't make any difference, but I
> guess your arm pmdp_set_access_flag is different.
We use the generic code, which ignores the dirty argument. Still, we should
pass the correct value through anyway, so I'll fix that too.
> However it seems "dirty" means write access and so the invocation
> would better match the pte case:
>
> if (pmdp_set_access_flags(vma, address & HPAGE_PMD_MASK, pmd, entry,
> flags & FAULT_FLAG_WRITE))
>
>
> But note, you still have to update it even when "dirty" == 0, or it'll
> still infinite loop for read accesses.
Yup. v2 to follow once we've re-run our testing.
Cheers,
Will
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