Messages in this thread | | | Date | Wed, 30 Nov 2022 17:11:36 +0100 | Subject | Re: [PATCH 03/10] mm/hugetlb: Document huge_pte_offset usage | From | David Hildenbrand <> |
| |
On 30.11.22 17:09, Peter Xu wrote: > On Wed, Nov 30, 2022 at 11:24:34AM +0100, David Hildenbrand wrote: >> On 29.11.22 20:35, Peter Xu wrote: >>> huge_pte_offset() is potentially a pgtable walker, looking up pte_t* for a >>> hugetlb address. >>> >>> Normally, it's always safe to walk a generic pgtable as long as we're with >>> the mmap lock held for either read or write, because that guarantees the >>> pgtable pages will always be valid during the process. >>> >>> But it's not true for hugetlbfs, especially shared: hugetlbfs can have its >>> pgtable freed by pmd unsharing, it means that even with mmap lock held for >>> current mm, the PMD pgtable page can still go away from under us if pmd >>> unsharing is possible during the walk. >>> >>> So we have two ways to make it safe even for a shared mapping: >>> >>> (1) If we're with the hugetlb vma lock held for either read/write, it's >>> okay because pmd unshare cannot happen at all. >>> >>> (2) If we're with the i_mmap_rwsem lock held for either read/write, it's >>> okay because even if pmd unshare can happen, the pgtable page cannot >>> be freed from under us. >>> >>> Document it. >>> >>> Signed-off-by: Peter Xu <peterx@redhat.com> >>> --- >>> include/linux/hugetlb.h | 32 ++++++++++++++++++++++++++++++++ >>> 1 file changed, 32 insertions(+) >>> >>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h >>> index 551834cd5299..81efd9b9baa2 100644 >>> --- a/include/linux/hugetlb.h >>> +++ b/include/linux/hugetlb.h >>> @@ -192,6 +192,38 @@ extern struct list_head huge_boot_pages; >>> pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma, >>> unsigned long addr, unsigned long sz); >>> +/* >>> + * huge_pte_offset(): Walk the hugetlb pgtable until the last level PTE. >>> + * Returns the pte_t* if found, or NULL if the address is not mapped. >>> + * >>> + * Since this function will walk all the pgtable pages (including not only >>> + * high-level pgtable page, but also PUD entry that can be unshared >>> + * concurrently for VM_SHARED), the caller of this function should be >>> + * responsible of its thread safety. One can follow this rule: >>> + * >>> + * (1) For private mappings: pmd unsharing is not possible, so it'll >>> + * always be safe if we're with the mmap sem for either read or write. >>> + * This is normally always the case, IOW we don't need to do anything >>> + * special. >> >> Maybe worth mentioning that hugetlb_vma_lock_read() and friends already >> optimize for private mappings, to not take the VMA lock if not required. > > Yes we can. I assume this is not super urgent so I'll hold a while to see > whether there's anything else that needs amending for the documents. > > Btw, even with hugetlb_vma_lock_read() checking SHARED for a private only > code path it's still better to not take the lock at all, because that still > contains a function jump which will be unnecesary.
IMHO it makes coding a lot more consistent and less error-prone when not care about whether to the the lock or not (as an optimization) and just having this handled "automatically".
Optimizing a jump out would rather smell like a micro-optimization.
-- Thanks,
David / dhildenb
| |