Messages in this thread |  | | Subject | Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork | From | John Hubbard <> | Date | Mon, 6 Apr 2020 14:30:07 -0700 |
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On 4/3/20 4:29 AM, Kirill A. Shutemov wrote: > The page can be included into collapse as long as it doesn't have extra > pins (from GUP or otherwise).
Hi Kirill,
s/callapse/collapse/ in the Subject line.
The commit message should mention that you're also removing a VM_BUG_ON_PAGE().
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> > --- > mm/khugepaged.c | 25 ++++++++++++++----------- > 1 file changed, 14 insertions(+), 11 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 57ff287caf6b..1e7e6543ebca 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -581,11 +581,18 @@ 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 > + * additinal pin on the page.
I'd recommend this wording instead, for the last two lines:
* from the page table tree. Therefore, this page will not * normally receive any additional pins.
> + * > + * 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 (total_mapcount(page) + PageSwapCache(page) != > + page_count(page)) {
I think it's time to put that logic ( "does this page have any extra references") into a small function. It's already duplicated once below. And the documentation is duplicated as well.
I took a quick peek at this patch because, after adding pin_user_pages*() APIs earlier to complement get_user_pages*(), I had a moment of doubt here: what if I'd done it in a way that required additional logic here? Fortunately, that's not the case: all pin_user_pages() calls on huge pages take a "primary/real" refcount, in addition to scribbling into the compound_pincount_ptr() area. whew. :)
> unlock_page(page); > result = SCAN_PAGE_COUNT; > goto out; > @@ -672,7 +679,6 @@ 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 > @@ -1206,12 +1212,9 @@ 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 (total_mapcount(page) + PageSwapCache(page) != > + page_count(page)) {> result = SCAN_PAGE_COUNT; > goto out_unmap; > } >
thanks, -- John Hubbard NVIDIA
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