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khugepaged: allow to collapse a page shared across fork

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The page can be included into collapse as long as it doesn't have extra
pins (from GUP or otherwise).

Logic to check the refcount is moved to a separate function.  For pages in
swap cache, add compound_nr(page) to the expected refcount, in order to
handle the compound page case.  This is in 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invariant it checks is no longer valid: the source can be mapped multiple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.

[yang.shi@linux.alibaba.com: remove error message when checking external pins]
  Link: http://lkml.kernel.org/r/1589317383-9595-1-git-send-email-yang.shi@linux.alibaba.com
[cai@lca.pw: fix set-but-not-used warning]
  Link: http://lkml.kernel.org/r/20200521145644.GA6367@ovpn-112-192.phx2.redhat.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Link: http://lkml.kernel.org/r/20200416160026.16538-6-kirill.shutemov@linux.intel.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Kirill A. Shutemov 2020-06-03 16:00:20 -07:00 committed by Linus Torvalds
parent ae2c5d8042
commit 9445689f3b
1 changed files with 37 additions and 9 deletions
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46
mm/khugepaged.c
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@ -526,6 +526,17 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte)
}
}
static bool is_refcount_suitable(struct page *page)
{
int expected_refcount;
expected_refcount = total_mapcount(page);
if (PageSwapCache(page))
expected_refcount += compound_nr(page);
return page_count(page) == expected_refcount;
}
static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
unsigned long address,
pte_t *pte)
@ -578,11 +589,17 @@ 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 from this process. 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;
@ -669,7 +686,6 @@ 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
@ -1221,11 +1237,23 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
}
/*
* 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.
*
* Here the check is racy it may see totmal_mapcount > refcount
* in some cases.
* For example, one process with one forked child process.
* The parent has the PMD split due to MADV_DONTNEED, then
* the child is trying unmap the whole PMD, but khugepaged
* may be scanning the parent between the child has
* PageDoubleMap flag cleared and dec the mapcount. So
* khugepaged may see total_mapcount > refcount.
*
* But such case is ephemeral we could always retry collapse
* later. However it may report false positive if the page
* has excessive GUP pins (i.e. 512). Anyway the same check
* will be done again later the risk seems low.
*/
if (page_count(page) != 1 + PageSwapCache(page)) {
if (!is_refcount_suitable(page)) {
result = SCAN_PAGE_COUNT;
goto out_unmap;
}