Messages in this thread | | | From | Christophe Leroy <> | Subject | Re: [PATCH] powerpc/64e: Fix wrong test in __ptep_test_and_clear_young() | Date | Tue, 5 Sep 2023 04:46:47 +0000 |
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Le 05/09/2023 à 04:36, Michael Ellerman a écrit : > Christophe Leroy <christophe.leroy@csgroup.eu> writes: >> Commit 45201c879469 ("powerpc/nohash: Remove hash related code from >> nohash headers.") replaced: >> >> if ((pte_val(*ptep) & (_PAGE_ACCESSED | _PAGE_HASHPTE)) == 0) >> return 0; >> >> By: >> >> if (pte_young(*ptep)) >> return 0; >> >> But it should be: >> >> if (!pte_young(*ptep)) >> return 0; > > > That seems bad :) > > But I don't know off the top of my head where > __ptep_test_and_clear_young() is used, and so what the symptoms could > be. Presumably nothing too bad or someone would have noticed? >
The two uses in mm/vmscan.c are as follows:
if (!ptep_test_and_clear_young(args->vma, addr, pte + i)) VM_WARN_ON_ONCE(true);
So it seems to be expected to never happen.
The only useful place it is used seems to be folio_check_references() which is part of the reclaim process. So probably it messes up swap, but to what extent ? ppc64e is for embedded systems. Do embedded systems have swap at all ?
Also surprising that it is also called from mm/debug_vm_pgtable.c so shouldn't we have noticed earlier ? I'll check if it works.
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