Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 20 Mar 2024 10:29:36 +0100 | Subject | Re: [PATCH 01/12] mm/ksm: catch tail page abnormal in page_stable_node | From | David Hildenbrand <> |
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On 20.03.24 10:05, Alex Shi wrote: > > > On 3/18/24 8:25 PM, David Hildenbrand wrote: >> On 18.03.24 13:14, alexs@kernel.org wrote: >>> From: Alex Shi <alexs@kernel.org> >>> >>> commit 19138349ed59 ("mm/migrate: Add folio_migrate_flags()") change the >>> meaning of func page_stable_node() to check the compound head for tail >>> 'page' instead of tail page self. >>> But seems both semantics are same at results, the func always return NULL >>> for tail page. So adding a bug monitor here in case of abnormal. >>> >>> Signed-off-by: Alex Shi <alexs@kernel.org> >>> Cc: Izik Eidus <izik.eidus@ravellosystems.com> >>> Cc: Matthew Wilcox <willy@infradead.org> >>> Cc: Andrea Arcangeli <aarcange@redhat.com> >>> Cc: Hugh Dickins <hughd@google.com> >>> Cc: Chris Wright <chrisw@sous-sol.org> >>> To: linux-kernel@vger.kernel.org >>> To: linux-mm@kvack.org >>> To: Andrew Morton <akpm@linux-foundation.org> >>> --- >>> mm/ksm.c | 7 ++++++- >>> 1 file changed, 6 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/ksm.c b/mm/ksm.c >>> index 8c001819cf10..3ff469961927 100644 >>> --- a/mm/ksm.c >>> +++ b/mm/ksm.c >>> @@ -1091,7 +1091,12 @@ static inline struct ksm_stable_node *folio_stable_node(struct folio *folio) >>> static inline struct ksm_stable_node *page_stable_node(struct page *page) >>> { >>> - return folio_stable_node(page_folio(page)); >>> + struct ksm_stable_node *node; >>> + >>> + node = folio_stable_node(page_folio(page)); >>> + VM_BUG_ON_PAGE(PageTail(page) && node, page); >> >> I don't really understand why we would want this. >> >> Only KSM folios can have a node in the stable tree. KSM folios cannot be large folios. At that is precisely what folio_stable_node() checks. >> >> If we'd have a large folio identify as a KSM folio we'd be in much bigger trouble. >> >> >> Besides, I'm sure you read "22) Do not crash the kernel" in Documentation/process/coding-style.rst >> > > Hi David, > > Thanks for comments! > Forgive my stupidity, I understand KSM stable tree has no compound pages, but when searching a tail page in ksm_do_scan(), why we couldn't be in a race, that another VM doing THP collapse on the same contents pages, while the 3rd vm is doing hugepage spliting?
We always call cmp_and_merge_page() while holding a reference on the page.
There, we call page_stable_node() directly and via stable_tree_search()->page_stable_node() on that page.
When stable_tree_search() returns a kpage, we also hold a reference to that kpage. So calling page_stable_node() on the kpage behaves the same.
As we are holding page references, pages cannot be split/merged and we should not see any races in page_stable_node().
Am I missing something?
Note that your change would also not help here: if it would be racy, you'd also not reliably catch any tail pages.
But it should not be racy unless I am missing something.
-- Cheers,
David / dhildenb
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