On Wed, Mar 23, 2022 at 4:29 PM Jiaqi Yan <jiaqiyan@xxxxxxxxxx> wrote:
>
> Make __collapse_huge_page_copy return whether
> collapsing/copying anonymous pages succeeded,
> and make collapse_huge_page handle the return status.
>
> Break existing PTE scan loop into two for-loops.
> The first loop copies source pages into target huge page,
> and can fail gracefully when running into memory errors in
> source pages. Roll back the page table and page states
> in the 2nd loop when copy failed:
> 1) re-establish the PTEs-to-PMD connection.
> 2) release pages back to their LRU list.
>
> Signed-off-by: Jiaqi Yan <jiaqiyan@xxxxxxxxxx>
> ---
> include/linux/highmem.h | 19 ++++++
> mm/khugepaged.c | 136 ++++++++++++++++++++++++++++++----------
> 2 files changed, 122 insertions(+), 33 deletions(-)
>
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 39bb9b47fa9c..15d0aa4d349c 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -281,6 +281,25 @@ static inline void copy_user_highpage(struct page *to, struct page *from,
> kunmap_local(vfrom);
> }
>
> +/*
> + * Machine check exception handled version of copy_user_highpage.
> + * Return true if copying page content failed; otherwise false.
> + */
> +static inline bool copy_user_highpage_mc(struct page *to, struct page *from,
> + unsigned long vaddr, struct vm_area_struct *vma)
Patch #2 defined copy_highpage_mc() which has the same implementation.
This function has two unused parameters: vaddr and vma. It seems we
just need to keep one.
> +{
> + char *vfrom, *vto;
> + unsigned long ret;
> +
> + vfrom = kmap_local_page(from);
> + vto = kmap_local_page(to);
> + ret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);
> + kunmap_local(vto);
> + kunmap_local(vfrom);
> +
> + return ret > 0;
> +}
> +
> #endif
>
> #ifndef __HAVE_ARCH_COPY_HIGHPAGE
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 131492fd1148..84ed177f56ff 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -739,44 +739,97 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> return 0;
> }
>
> -static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> - struct vm_area_struct *vma,
> - unsigned long address,
> - spinlock_t *ptl,
> - struct list_head *compound_pagelist)
> +/*
> + * __collapse_huge_page_copy - attempts to copy memory contents from normal
> + * pages to a hugepage. Cleanup the normal pages if copy succeeds;
> + * otherwise restore the original pmd page table.
> + *
> + * @pte: starting of the PTEs to copy from
> + * @page: the new hugepage to copy contents to
> + * @pmd: pointer to the new hugepage's PMD
> + * @rollback: the original normal PTEs' PMD
> + * @address: starting address to copy
> + * @pte_ptl: lock on normal pages' PTEs
> + * @compound_pagelist: list that stores compound pages
> + */
> +static bool __collapse_huge_page_copy(pte_t *pte,
> + struct page *page,
> + pmd_t *pmd,
> + pmd_t rollback,
> + struct vm_area_struct *vma,
> + unsigned long address,
> + spinlock_t *pte_ptl,
> + struct list_head *compound_pagelist)
> {
> struct page *src_page, *tmp;
> pte_t *_pte;
> - for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> - _pte++, page++, address += PAGE_SIZE) {
> - pte_t pteval = *_pte;
> + pte_t pteval;
> + unsigned long _address;
> + spinlock_t *pmd_ptl;
> + bool copy_succeeded = true;
>
> - if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> + /*
> + * Copying pages' contents is subject to memory poison at any iteration.
> + */
> + for (_pte = pte, _address = address;
> + _pte < pte + HPAGE_PMD_NR;
> + _pte++, page++, _address += PAGE_SIZE) {
> + pteval = *_pte;
> +
> + if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval)))
> clear_user_highpage(page, address);
> - add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> - if (is_zero_pfn(pte_pfn(pteval))) {
> - /*
> - * ptl mostly unnecessary.
> - */
> - spin_lock(ptl);
> - ptep_clear(vma->vm_mm, address, _pte);
> - spin_unlock(ptl);
> + else {
> + src_page = pte_page(pteval);
> + if (copy_user_highpage_mc(page, src_page, address, vma)) {
> + copy_succeeded = false;
> + break;
> + }
> + }
> + }
> +
> + if (!copy_succeeded) {
> + /*
> + * Copying failed, re-establish the regular PMD that
> + * points to regular page table. Since PTEs are still
> + * isolated and locked, acquiring anon_vma_lock is unnecessary.
> + */
> + pmd_ptl = pmd_lock(vma->vm_mm, pmd);
> + pmd_populate(vma->vm_mm, pmd, pmd_pgtable(rollback));
> + spin_unlock(pmd_ptl);
I think we could just jump to list_for_each_entry_safe, right? But it
seems that list_for_each_entry_safe is not enough, please see the
later comments.
> + }
> +
> + for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
> + _pte++, _address += PAGE_SIZE) {
> + pteval = *_pte;
> + if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> + if (copy_succeeded) {
With the above jump, you don't need to check if copy is succeeded or
not since this is the succeeded path only.
> + add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> + if (is_zero_pfn(pte_pfn(pteval))) {
> + /*
> + * ptl mostly unnecessary.
> + */
> + spin_lock(pte_ptl);
> + pte_clear(vma->vm_mm, _address, _pte);
> + spin_unlock(pte_ptl);
> + }
> }
> } else {
> src_page = pte_page(pteval);
> - copy_user_highpage(page, src_page, address, vma);
> if (!PageCompound(src_page))
> release_pte_page(src_page);
> - /*
> - * ptl mostly unnecessary, but preempt has to
> - * be disabled to update the per-cpu stats
> - * inside page_remove_rmap().
> - */
> - spin_lock(ptl);
> - ptep_clear(vma->vm_mm, address, _pte);
> - page_remove_rmap(src_page, false);
> - spin_unlock(ptl);
> - free_page_and_swap_cache(src_page);
> +
> + if (copy_succeeded) {
> + /*
> + * ptl mostly unnecessary, but preempt has to
> + * be disabled to update the per-cpu stats
> + * inside page_remove_rmap().
> + */
> + spin_lock(pte_ptl);
> + pte_clear(vma->vm_mm, _address, _pte);
> + page_remove_rmap(src_page, false);
> + spin_unlock(pte_ptl);
> + free_page_and_swap_cache(src_page);
> + }
> }
> }
>
> @@ -784,6 +837,8 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
> list_del(&src_page->lru);
> release_pte_page(src_page);
> }
If copy failed you need to unpin the isolated pages and put them back
to LRU IIUC.
> +
> + return copy_succeeded;
> }
>
> static void khugepaged_alloc_sleep(void)
> @@ -1066,6 +1121,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> struct vm_area_struct *vma;
> struct mmu_notifier_range range;
> gfp_t gfp;
> + bool copy_succeeded = false;
>
> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>
> @@ -1177,9 +1233,13 @@ static void collapse_huge_page(struct mm_struct *mm,
> */
> anon_vma_unlock_write(vma->anon_vma);
>
> - __collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl,
> - &compound_pagelist);
> + copy_succeeded = __collapse_huge_page_copy(pte, new_page, pmd, _pmd,
> + vma, address, pte_ptl, &compound_pagelist);
> pte_unmap(pte);
> + if (!copy_succeeded) {
Shall the fail handling be moved before pte_unmap()?
> + result = SCAN_FAIL;
I think a new result is preferred, for example, SCAN_COPY_MC, it would
be helpful for debug.
> + goto out_up_write;
> + }
> /*
> * spin_lock() below is not the equivalent of smp_wmb(), but
> * the smp_wmb() inside __SetPageUptodate() can be reused to
> @@ -1364,9 +1424,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> pte_unmap_unlock(pte, ptl);
> if (ret) {
> node = khugepaged_find_target_node();
> - /* collapse_huge_page will return with the mmap_lock released */
> - collapse_huge_page(mm, address, hpage, node,
> - referenced, unmapped);
> + /*
> + * collapse_huge_page will return with the mmap_r+w_lock released.
> + * It is uncertain if *hpage is NULL or not when collapse_huge_page
> + * returns, so keep ret=1 to jump to breakouterloop_mmap_lock
> + * in khugepaged_scan_mm_slot, then *hpage will be freed
> + * if collapse failed.
It may be not true for !NUMA case, the huge page may be reused, so you
need to make sure the huge page is cleared before return, I had a
patch that remove the special case for !NUMA
(https://lore.kernel.org/linux-mm/20210817202146.3218-1-shy828301@xxxxxxxxx/),
it could avoid clearing huge page, I should resurrect it.
> + */
> + collapse_huge_page(mm, address, hpage, node, referenced, unmapped);
> }
> out:
> trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
> @@ -2168,6 +2233,11 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
> khugepaged_scan_file(mm, file, pgoff, hpage);
> fput(file);
> } else {
> + /*
> + * mmap_read_lock is
> + * 1) released if both scan and collapse succeeded;
> + * 2) still held if either scan or collapse failed.
> + */
> ret = khugepaged_scan_pmd(mm, vma,
> khugepaged_scan.address,
> hpage);
> --
> 2.35.1.894.gb6a874cedc-goog
>
