Re: [PATCH 5/7] mm: free user PTE page table pages

[Qi Zheng]

On 7/18/21 2:19 PM, Mika Penttilä wrote:

> > +
> > +/*
> > + * returns true if the pmd has been populated with PTE page table,
> > + * or false for all other cases.
> > + */
> > +bool pte_install_try_get(struct mm_struct *mm, pmd_t *pmd, pgtable_t 
> > *pte)
> > +{
> > +    spinlock_t *ptl;
> > +    bool retval = true;
> > +
> > +retry:
> > +    ptl = pmd_lock(mm, pmd);
> > +    if (likely(pmd_none(*pmd))) {
> > +        __pte_install(mm, pmd, pte);
> > +    } else if (pmd_leaf(*pmd) || !pmd_present(*pmd)) {
> > +        retval = false;
> > +    } else if (!pte_get_unless_zero(pmd)) {
> > +        spin_unlock(ptl);
> > +        goto retry;
> > +    }
> > +    spin_unlock(ptl);
> > +    return retval;
> > +}
> > +
>
> Can pte_get_unless_zero() return true above? Can the pmd have been by 
> populated by others? In that case the ref count is wrongly incremented.

Here we only have mmap_read_lock(mm), so the pmd can be populated with
other PTE page table page after a page fault in a different thread B of 
this mm. In this case, thread B already hold a pte_refcount of the PTE 
page table page populated in the pmd, so pte_get_unless_zero() can
return true above.

Similarly, if THP is enabled, the pmd also can be populated with a THP 
page, we can see more detail in comment in handle_pte_fault(). The
pmd_leaf() above is to detect this situation.