On Wed 22-08-18 09:48:16, Mike Kravetz wrote:
> On 08/22/2018 05:28 AM, Michal Hocko wrote:
> > On Tue 21-08-18 18:10:42, Mike Kravetz wrote:
> > [...]
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index eb477809a5c0..8cf853a4b093 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -1362,11 +1362,21 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> >> }
> >>
> >> /*
> >> - * We have to assume the worse case ie pmd for invalidation. Note that
> >> - * the page can not be free in this function as call of try_to_unmap()
> >> - * must hold a reference on the page.
> >> + * For THP, we have to assume the worse case ie pmd for invalidation.
> >> + * For hugetlb, it could be much worse if we need to do pud
> >> + * invalidation in the case of pmd sharing.
> >> + *
> >> + * Note that the page can not be free in this function as call of
> >> + * try_to_unmap() must hold a reference on the page.
> >> */
> >> end = min(vma->vm_end, start + (PAGE_SIZE << compound_order(page)));
> >> + if (PageHuge(page)) {
> >> + /*
> >> + * If sharing is possible, start and end will be adjusted
> >> + * accordingly.
> >> + */
> >> + (void)huge_pmd_sharing_possible(vma, &start, &end);
> >> + }
> >> mmu_notifier_invalidate_range_start(vma->vm_mm, start, end);
> >
> > I do not get this part. Why don't we simply unconditionally invalidate
> > the whole huge page range?
>
> In this routine, we are only unmapping a single page. The existing code
> is limiting the invalidate range to that page size: 4K or 2M. With shared
> PMDs, we have the possibility of unmapping a PUD_SIZE area: 1G. I don't
> think we want to unconditionally invalidate 1G. Is that what you are asking?
But we know that huge_pmd_unshare unmapped a shared pte so we know when
to flush 2MB or 1GB. I really do not like how huge_pmd_sharing_possible
a) duplicates some checks and b) it updates start/stop out of line.
> I do not know how often PMD sharing is exercised. It certainly is used by
> DBs for large shared areas. I suspect it is less frequent than hugtlb pages
> in general, and certainly less frequent than THP or base pages.
>
> >>
> >> while (page_vma_mapped_walk(&pvmw)) {
> >> @@ -1409,6 +1419,32 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> >> subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
> >> address = pvmw.address;
> >>
> >> + if (PageHuge(page)) {
> >> + if (huge_pmd_unshare(mm, &address, pvmw.pte)) {
> >
> > huge_pmd_unshare is documented to require a pte lock. Where do we take
> > it?
>
> It is somewhat hidden, but we are in the loop:
>
> while (page_vma_mapped_walk(&pvmw)) {
>
> The routine page_vma_mapped_walk will acquire the lock, and it correctly
> checks for huge pages and uses huge_pte_lockptr().
>
> page_vma_mapped_walk_done() will release the lock.
OK, I can see it now. Thanks for the clarification. page_vma_mapped_walk
is quite hard to follow.
--
Michal Hocko
SUSE Labs
