On 25.04.24 09:27, Lance Yang wrote:
On Thu, Apr 25, 2024 at 3:21 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 25.04.24 05:45, Lance Yang wrote:
Hey Zi,
On Thu, Apr 25, 2024 at 6:46 AM Zi Yan <zi.yan@xxxxxxxx> wrote:
From: Zi Yan <ziy@xxxxxxxxxx>
In __folio_remove_rmap(), a large folio is added to deferred split list
if any page in a folio loses its final mapping. It is possible that
the folio is unmapped fully, but it is unnecessary to add the folio
Agreed. If a folio is fully unmapped, then that's unnecessary to add
to the deferred split list.
to deferred split list at all. Fix it by checking folio->_nr_pages_mapped
before adding a folio to deferred split list. If the folio is already
on the deferred split list, it will be skipped. This issue applies to
both PTE-mapped THP and mTHP.
Commit 98046944a159 ("mm: huge_memory: add the missing
folio_test_pmd_mappable() for THP split statistics") tried to exclude
mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
deferred_split_folio() the order-9 folio is folio_test_pmd_mappable().
However, this miscount was present even earlier due to implementation,
since PTEs are unmapped individually and first PTE unmapping adds the THP
into the deferred split list.
With commit b06dc281aa99 ("mm/rmap: introduce
folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapped
folios in one shot without causing the miscount, hence this patch.
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
---
mm/rmap.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index a7913a454028..2809348add7b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1553,9 +1553,10 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
* page of the folio is unmapped and at least one page
* is still mapped.
*/
- if (folio_test_large(folio) && folio_test_anon(folio))
- if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
- deferred_split_folio(folio);
+ if (folio_test_large(folio) && folio_test_anon(folio) &&
+ ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
+ (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
Perhaps we only need to check the mapcount?
IIUC, if a large folio that was PMD/PTE mapped is fully unmapped here,
then folio_mapcount() will return 0.
See discussion on v1. folio_large_mapcount() would achieve the same
without another folio_test_large() check, but in the context of this
patch it doesn't really matter.
Got it. Thanks for pointing that out!
I'll take a closer look at the discussion in v1.
Forgot to add: as long as the large mapcount patches are not upstream,
folio_large_mapcount() would be expensive. So this patch can be added
independent of the other stuff.
--
Cheers,
David / dhildenb
