On Thu, Mar 24, 2022 at 7:20 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> 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.
>
Version 2 will delete copy_user_highpage_mc() from the patch series,
and just keep copy_highpage_mc().
> > +{
> > + 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.
Yes, it is not enough, we need to release_pte_page() for non-compound pages.
If we are going to use jump/goto, we need two for-loops over PTEs, one
for the copy
succeeded case, and one for the failure case. What I have right now is combining
the two for-loops into one.
I don't like to switch to jump because: 1) goto statements have
bad readability, 2) one for-loop is more concise than two for-loops.
>
> > + }
> > +
> > + 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.
No matter if the copy failed or succeeded, we need to use a for-loop
to unpin isolated pages
and put them back to LRU (i.e. release_pte_page()). In current code,
this is a common
operation in the above for-loop.
>
> > +
> > + 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()?
I don't think so. I think at this point we need to pte_unmap() no
matter if the copying
succeeded or not. It is obviously needed if __collapse_huge_page_copy succeeded
(because existing code already assumes __collapse_huge_page_copy
always succeeded).
For the failure case, my reasoning is based on __collapse_huge_page_isolate.
If __collapse_huge_page_isolate failed, we need to pte_unmap()
before goto out_up_write. So I would assume if __collapse_huge_page_copy failed,
pte_unmap() is also required.
>
> > + result = SCAN_FAIL;
>
> I think a new result is preferred, for example, SCAN_COPY_MC, it would
> be helpful for debug.
Version 2 will add this new enum.
>
> > + 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.
Thanks for catching this! Clearing the hugepage before returning is
for security reasons
or to avoid memory corruption, right?
If my understanding is correct, once your patch is merged, this will
be good. Your patch
seems to be a very simple one, do you expect it to be merged soon?
>
> > + */
> > + 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
> >
Thanks for the quick and valuable feedback.
I will send out the v2 RFC, and please leave/reply there. Thanks!
