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SubjectRe: [PATCH v4 1/3] mm/khugepaged: refactor collapse_file control flow
On Tue, Feb 21, 2023 at 1:54 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Fri, Feb 17, 2023 at 05:54:37PM +0900, David Stevens wrote:
> > From: David Stevens <stevensd@chromium.org>
> >
> > Add a rollback label to deal with failure, instead of continuously
> > checking for RESULT_SUCCESS, to make it easier to add more failure
> > cases. The refactoring also allows the collapse_file tracepoint to
> > include hpage on success (instead of NULL).
> >
> > Signed-off-by: David Stevens <stevensd@chromium.org>
> > ---
> > mm/khugepaged.c | 223 ++++++++++++++++++++++++------------------------
> > 1 file changed, 110 insertions(+), 113 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 8dbc39896811..6a3d6d2e25e0 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1885,6 +1885,12 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> > if (result != SCAN_SUCCEED)
> > goto out;
> >
> > + __SetPageLocked(hpage);
> > + if (is_shmem)
> > + __SetPageSwapBacked(hpage);
> > + hpage->index = start;
> > + hpage->mapping = mapping;
> > +
> > /*
> > * Ensure we have slots for all the pages in the range. This is
> > * almost certainly a no-op because most of the pages must be present
> > @@ -1897,16 +1903,10 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> > xas_unlock_irq(&xas);
> > if (!xas_nomem(&xas, GFP_KERNEL)) {
> > result = SCAN_FAIL;
> > - goto out;
> > + goto rollback;
> > }
> > } while (1);
> >
> > - __SetPageLocked(hpage);
> > - if (is_shmem)
> > - __SetPageSwapBacked(hpage);
> > - hpage->index = start;
> > - hpage->mapping = mapping;
> > -
> > /*
> > * At this point the hpage is locked and not up-to-date.
> > * It's safe to insert it into the page cache, because nobody would
> > @@ -2123,131 +2123,128 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> > */
> > try_to_unmap_flush();
> >
> > - if (result == SCAN_SUCCEED) {
> > - /*
> > - * Replacing old pages with new one has succeeded, now we
> > - * attempt to copy the contents.
> > - */
> > - index = start;
> > - list_for_each_entry(page, &pagelist, lru) {
> > - while (index < page->index) {
> > - clear_highpage(hpage + (index % HPAGE_PMD_NR));
> > - index++;
> > - }
> > - if (copy_mc_page(hpage + (page->index % HPAGE_PMD_NR),
> > - page) > 0) {
> > - result = SCAN_COPY_MC;
> > - break;
> > - }
> > - index++;
> > - }
> > - while (result == SCAN_SUCCEED && index < end) {
> > + if (result != SCAN_SUCCEED)
> > + goto rollback;
> > +
> > + /*
> > + * Replacing old pages with new one has succeeded, now we
> > + * attempt to copy the contents.
> > + */
> > + index = start;
> > + list_for_each_entry(page, &pagelist, lru) {
> > + while (index < page->index) {
> > clear_highpage(hpage + (index % HPAGE_PMD_NR));
> > index++;
> > }
> > + if (copy_mc_page(hpage + (page->index % HPAGE_PMD_NR),
> > + page) > 0) {
> > + result = SCAN_COPY_MC;
> > + goto rollback;
> > + }
> > + index++;
> > + }
> > + while (index < end) {
> > + clear_highpage(hpage + (index % HPAGE_PMD_NR));
> > + index++;
> > }
> >
> > - if (result == SCAN_SUCCEED) {
> > - /*
> > - * Copying old pages to huge one has succeeded, now we
> > - * need to free the old pages.
> > - */
> > - list_for_each_entry_safe(page, tmp, &pagelist, lru) {
> > - list_del(&page->lru);
> > - page->mapping = NULL;
> > - page_ref_unfreeze(page, 1);
> > - ClearPageActive(page);
> > - ClearPageUnevictable(page);
> > - unlock_page(page);
> > - put_page(page);
> > - }
> > + /*
> > + * Copying old pages to huge one has succeeded, now we
> > + * need to free the old pages.
> > + */
> > + list_for_each_entry_safe(page, tmp, &pagelist, lru) {
> > + list_del(&page->lru);
> > + page->mapping = NULL;
> > + page_ref_unfreeze(page, 1);
> > + ClearPageActive(page);
> > + ClearPageUnevictable(page);
> > + unlock_page(page);
> > + put_page(page);
> > + }
> >
> > - xas_lock_irq(&xas);
> > - if (is_shmem)
> > - __mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
> > - else
> > - __mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
> > + xas_lock_irq(&xas);
> > + if (is_shmem)
> > + __mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
> > + else
> > + __mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
> > +
> > + if (nr_none) {
> > + __mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
> > + /* nr_none is always 0 for non-shmem. */
> > + __mod_lruvec_page_state(hpage, NR_SHMEM, nr_none);
> > + }
> > + /* Join all the small entries into a single multi-index entry. */
> > + xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> > + xas_store(&xas, hpage);
> > + xas_unlock_irq(&xas);
> >
> > - if (nr_none) {
> > - __mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
> > - /* nr_none is always 0 for non-shmem. */
> > - __mod_lruvec_page_state(hpage, NR_SHMEM, nr_none);
> > - }
> > - /* Join all the small entries into a single multi-index entry. */
> > - xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> > - xas_store(&xas, hpage);
> > - xas_unlock_irq(&xas);
> > + folio = page_folio(hpage);
> > + folio_mark_uptodate(folio);
> > + folio_ref_add(folio, HPAGE_PMD_NR - 1);
> >
> > - folio = page_folio(hpage);
> > - folio_mark_uptodate(folio);
> > - folio_ref_add(folio, HPAGE_PMD_NR - 1);
> > + if (is_shmem)
> > + folio_mark_dirty(folio);
> > + folio_add_lru(folio);
> >
> > - if (is_shmem)
> > - folio_mark_dirty(folio);
> > - folio_add_lru(folio);
> > + /*
> > + * Remove pte page tables, so we can re-fault the page as huge.
> > + */
> > + result = retract_page_tables(mapping, start, mm, addr, hpage,
> > + cc);
> > + unlock_page(hpage);
> > + goto out;
> > +
> > +rollback:
> > + /* Something went wrong: roll back page cache changes */
> > + xas_lock_irq(&xas);
> > + if (nr_none) {
> > + mapping->nrpages -= nr_none;
> > + shmem_uncharge(mapping->host, nr_none);
> > + }
> >
> > - /*
> > - * Remove pte page tables, so we can re-fault the page as huge.
> > - */
> > - result = retract_page_tables(mapping, start, mm, addr, hpage,
> > - cc);
> > - unlock_page(hpage);
> > - hpage = NULL;
> > - } else {
> > - /* Something went wrong: roll back page cache changes */
> > - xas_lock_irq(&xas);
> > - if (nr_none) {
> > - mapping->nrpages -= nr_none;
> > - shmem_uncharge(mapping->host, nr_none);
> > + xas_set(&xas, start);
> > + xas_for_each(&xas, page, end - 1) {
> > + page = list_first_entry_or_null(&pagelist,
> > + struct page, lru);
> > + if (!page || xas.xa_index < page->index) {
> > + if (!nr_none)
> > + break;
> > + nr_none--;
> > + /* Put holes back where they were */
> > + xas_store(&xas, NULL);
> > + continue;
> > }
> >
> > - xas_set(&xas, start);
> > - xas_for_each(&xas, page, end - 1) {
> > - page = list_first_entry_or_null(&pagelist,
> > - struct page, lru);
> > - if (!page || xas.xa_index < page->index) {
> > - if (!nr_none)
> > - break;
> > - nr_none--;
> > - /* Put holes back where they were */
> > - xas_store(&xas, NULL);
> > - continue;
> > - }
> > + VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
> >
> > - VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
> > + /* Unfreeze the page. */
> > + list_del(&page->lru);
> > + page_ref_unfreeze(page, 2);
> > + xas_store(&xas, page);
> > + xas_pause(&xas);
> > + xas_unlock_irq(&xas);
> > + unlock_page(page);
> > + putback_lru_page(page);
> > + xas_lock_irq(&xas);
> > + }
> > + VM_BUG_ON(nr_none);
> > + /*
> > + * Undo the updates of filemap_nr_thps_inc for non-SHMEM file only.
> > + * This undo is not needed unless failure is due to SCAN_COPY_MC.
> > + */
> > + if (!is_shmem && result == SCAN_COPY_MC)
> > + filemap_nr_thps_dec(mapping);
> >
> > - /* Unfreeze the page. */
> > - list_del(&page->lru);
> > - page_ref_unfreeze(page, 2);
> > - xas_store(&xas, page);
> > - xas_pause(&xas);
> > - xas_unlock_irq(&xas);
> > - unlock_page(page);
> > - putback_lru_page(page);
> > - xas_lock_irq(&xas);
> > - }
> > - VM_BUG_ON(nr_none);
> > - /*
> > - * Undo the updates of filemap_nr_thps_inc for non-SHMEM file only.
> > - * This undo is not needed unless failure is due to SCAN_COPY_MC.
> > - */
> > - if (!is_shmem && result == SCAN_COPY_MC)
> > - filemap_nr_thps_dec(mapping);
> > + xas_unlock_irq(&xas);
> >
> > - xas_unlock_irq(&xas);
> > + hpage->mapping = NULL;
> >
> > - hpage->mapping = NULL;
> > - }
> > + unlock_page(hpage);
> > + mem_cgroup_uncharge(page_folio(hpage));
> > + put_page(hpage);
> >
> > - if (hpage)
> > - unlock_page(hpage);
> > out:
> > VM_BUG_ON(!list_empty(&pagelist));
> > - if (hpage) {
> > - mem_cgroup_uncharge(page_folio(hpage));
> > - put_page(hpage);
> > - }
>
> Moving this chunk seems wrong, as it can leak the huge page if
> alloc_charge_hpage() failed on mem charging, iiuc.

Yeah, good catch.

>
> And I found that keeping it seems wrong either, because if mem charge
> failed we'll uncharge even without charging it before. But that's nothing
> about this patch alone.

We should be able to just simply check the return value. For example:

if (result == SCAN_CGROUP_CHARGE_FAIL)
put_page(hpage);

>
> Posted a patch for this:
>
> https://lore.kernel.org/all/20230221214344.609226-1-peterx@redhat.com/
>
> I _think_ this patch will make sense after rebasing to that fix if that's
> correct, but please review it and double check.

It is ok too.


>
> Thanks,
>
> > -
> > trace_mm_khugepaged_collapse_file(mm, hpage, index, is_shmem, addr, file, nr, result);
> > return result;
> > }
> > --
> > 2.39.2.637.g21b0678d19-goog
> >
>
> --
> Peter Xu
>

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