Messages in this thread | | | Subject | Re: [PATCHv3, RESEND 5/8] khugepaged: Allow to collapse a page shared across fork | From | John Hubbard <> | Date | Tue, 14 Apr 2020 19:43:33 -0700 |
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On 2020-04-13 05:52, Kirill A. Shutemov wrote: > The page can be included into collapse as long as it doesn't have extra > pins (from GUP or otherwise). > > Logic to check the refcound is moved to a separate function. > Note that the function is ready to deal with compound pages. It's > preparation for the following patch. > > VM_BUG_ON_PAGE() was removed from __collapse_huge_page_copy() as the > invariant it checks is no longer valid: the source can be mapped > multiple times now. > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> > --- > mm/khugepaged.c | 41 ++++++++++++++++++++++++++++++----------- > 1 file changed, 30 insertions(+), 11 deletions(-)
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
thanks, -- John Hubbard NVIDIA
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index e3e41c2768d8..f9864644c3b7 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -529,6 +529,24 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte) > } > } > > +static bool is_refcount_suitable(struct page *page) > +{ > + int expected_refcount, refcount; > + > + refcount = page_count(page); > + expected_refcount = total_mapcount(page); > + if (PageSwapCache(page)) > + expected_refcount += compound_nr(page); > + > + if (IS_ENABLED(CONFIG_DEBUG_VM) && expected_refcount > refcount) { > + pr_err("expected_refcount: %d, refcount: %d\n", > + expected_refcount, refcount); > + dump_page(page, "Unexpected refcount"); > + } > + > + return page_count(page) == expected_refcount; > +} > + > static int __collapse_huge_page_isolate(struct vm_area_struct *vma, > unsigned long address, > pte_t *pte) > @@ -581,11 +599,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, > } > > /* > - * cannot use mapcount: can't collapse if there's a gup pin. > - * The page must only be referenced by the scanned process > - * and page swap cache. > + * Check if the page has any GUP (or other external) pins. > + * > + * The page table that maps the page has been already unlinked > + * from the page table tree and this process cannot get > + * an additinal pin on the page. > + * > + * New pins can come later if the page is shared across fork, > + * but not for the this process. It is fine. The other process > + * cannot write to the page, only trigger CoW. > */ > - if (page_count(page) != 1 + PageSwapCache(page)) { > + if (!is_refcount_suitable(page)) { > unlock_page(page); > result = SCAN_PAGE_COUNT; > goto out; > @@ -672,7 +696,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page, > } else { > src_page = pte_page(pteval); > copy_user_highpage(page, src_page, address, vma); > - VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page); > release_pte_page(src_page); > /* > * ptl mostly unnecessary, but preempt has to > @@ -1201,12 +1224,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, > goto out_unmap; > } > > - /* > - * cannot use mapcount: can't collapse if there's a gup pin. > - * The page must only be referenced by the scanned process > - * and page swap cache. > - */ > - if (page_count(page) != 1 + PageSwapCache(page)) { > + /* Check if the page has any GUP (or other external) pins */ > + if (!is_refcount_suitable(page)) { > result = SCAN_PAGE_COUNT; > goto out_unmap; > } >
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