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.- 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?
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.