Re: [RFC PATCH v2 3/7] mm: migrate: allocate the right size of non hugetlb or THP compound pages.

On Fri, Dec 10, 2021 at 8:08 AM Zi Yan [off-list ref] wrote:

On 10 Dec 2021, at 2:53, Eric Ren wrote:

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Hi,

On 2021/12/10 07:04, Zi Yan wrote:

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From: Zi Yan <ziy@nvidia.com>

alloc_migration_target() is used by alloc_contig_range() and non-LRU
movable compound pages can be migrated. Current code does not allocate the
right page size for such pages. Check THP precisely using
is_transparent_huge() and add allocation support for non-LRU compound
pages.

Could you elaborate on why the current code doesn't get the right size?  how this patch fixes it.

The current code only check PageHuge() and PageTransHuge(). Non-LRU compound
pages will be regarded as PageTransHuge() and an order-9 page is always allocated
regardless of the actual page order. This patch makes the exact check for
THPs using is_transparent_huge() instead of PageTransHuge() and checks PageCompound()
after PageHuge() and is_transparent_huge() for non-LRU compound pages.

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The description sounds like it's an existing bug, if so, the patch subject should be changed to
"Fixes ..."?

I have not seen any related bug report.

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Signed-off-by: Zi Yan <ziy@nvidia.com>
---
  mm/migrate.c | 8 ++++++--
  1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index d487a399253b..2ce3c771b1de 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c

@@ -1563,7 +1563,7 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)
             return alloc_huge_page_nodemask(h, nid, mtc->nmask, gfp_mask);
     }
 -   if (PageTransHuge(page)) {
+    if (is_transparent_hugepage(page)) {
             /*
              * clear __GFP_RECLAIM to make the migration callback
              * consistent with regular THP allocations.

@@ -1572,13 +1572,17 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)

if (PageTransHuge(page)) {  // just give more code context
...

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             gfp_mask |= GFP_TRANSHUGE;
             order = HPAGE_PMD_ORDER;

order assigned here

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     }
+    if (PageCompound(page)) {
+            gfp_mask |= __GFP_COMP;
+            order = compound_order(page);

re-assinged again as THP is a compound page?

Ah, you are right. Will use else if instead. Thanks.

You don't have to use else if, you could just do:

if (PageCompound(page)) {
    gfp_mask |= __GFP_COMP;
    order = compound_order(page);
}

if (is_transparent_hugepage(page)) {
    /* Manipulate THP specific gfp mask */
    ....
}

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Thanks,
Eric

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+    }
     zidx = zone_idx(page_zone(page));
     if (is_highmem_idx(zidx) || zidx == ZONE_MOVABLE)
             gfp_mask |= __GFP_HIGHMEM;
     new_page = __alloc_pages(gfp_mask, order, nid, mtc->nmask);
 -   if (new_page && PageTransHuge(new_page))
+    if (new_page && is_transparent_hugepage(page))
             prep_transhuge_page(new_page);
     return new_page;

--
Best Regards,
Yan, Zi