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arm64/mm: improve comment in contpte_ptep_get_lockless()

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Make clear the atmicity/consistency requirements of the API and how we
achieve them.

Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/
Link: https://lkml.kernel.org/r/20240226120321.1055731-3-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Ryan Roberts 2024-02-26 12:03:21 +00:00 committed by Andrew Morton
parent 912609e96c
commit 94c18d5f7e
1 changed files with 14 additions and 10 deletions
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24
arch/arm64/mm/contpte.c
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@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get);
pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
{
/*
* Gather access/dirty bits, which may be populated in any of the ptes
* of the contig range. We may not be holding the PTL, so any contiguous
* range may be unfolded/modified/refolded under our feet. Therefore we
* ensure we read a _consistent_ contpte range by checking that all ptes
* in the range are valid and have CONT_PTE set, that all pfns are
* contiguous and that all pgprots are the same (ignoring access/dirty).
* If we find a pte that is not consistent, then we must be racing with
* an update so start again. If the target pte does not have CONT_PTE
* set then that is considered consistent on its own because it is not
* part of a contpte range.
* The ptep_get_lockless() API requires us to read and return *orig_ptep
* so that it is self-consistent, without the PTL held, so we may be
* racing with other threads modifying the pte. Usually a READ_ONCE()
* would suffice, but for the contpte case, we also need to gather the
* access and dirty bits from across all ptes in the contiguous block,
* and we can't read all of those neighbouring ptes atomically, so any
* contiguous range may be unfolded/modified/refolded under our feet.
* Therefore we ensure we read a _consistent_ contpte range by checking
* that all ptes in the range are valid and have CONT_PTE set, that all
* pfns are contiguous and that all pgprots are the same (ignoring
* access/dirty). If we find a pte that is not consistent, then we must
* be racing with an update so start again. If the target pte does not
* have CONT_PTE set then that is considered consistent on its own
* because it is not part of a contpte range.
*/
pgprot_t orig_prot;