Hi, Mike,
On Sun, Sep 18, 2022 at 07:13:48PM -0700, Mike Kravetz wrote:
> +struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> + unsigned long address, unsigned int flags)
> +{
> + struct hstate *h = hstate_vma(vma);
> + struct mm_struct *mm = vma->vm_mm;
> + unsigned long haddr = address & huge_page_mask(h);
> + struct page *page = NULL;
> + spinlock_t *ptl;
> + pte_t *pte, entry;
> +
> + /*
> + * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> + * follow_hugetlb_page().
> + */
> + if (WARN_ON_ONCE(flags & FOLL_PIN))
> + return NULL;
> +
> +retry:
> + /*
> + * vma lock prevents racing with another thread doing a pmd unshare.
> + * This keeps pte as returned by huge_pte_offset valid.
> + */
> + hugetlb_vma_lock_read(vma);
I'm not sure whether it's okay to take a rwsem here, as the code can be
called by e.g. FOLL_NOWAIT?
I'm wondering whether it's fine to just drop this anyway, just always walk
it lockless. IIUC gup callers should be safe here because the worst case
is the caller will fetch a wrong page, but then it should be invalidated
very soon with mmu notifiers. One thing worth mention is that pmd unshare
should never free a pgtable page.
IIUC it's also the same as fast-gup - afaiu we don't take the read vma lock
in fast-gup too but I also think it's safe. But I hope I didn't miss
something.
--
Peter Xu
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