On Wed, Nov 30, 2022 at 05:11:36PM +0100, David Hildenbrand wrote: > 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@xxxxxxxxxx> > > > > --- > > > > 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. Or we can move the lock helpers into the headers, too. -- Peter Xu
