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SubjectRe: [PATCH v9 07/24] mm: VMA sequence count
On Tue, 13 Mar 2018, Laurent Dufour wrote:

> diff --git a/mm/mmap.c b/mm/mmap.c
> index faf85699f1a1..5898255d0aeb 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -558,6 +558,10 @@ void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
> else
> mm->highest_vm_end = vm_end_gap(vma);
>
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + seqcount_init(&vma->vm_sequence);
> +#endif
> +
> /*
> * vma->vm_prev wasn't known when we followed the rbtree to find the
> * correct insertion point for that vma. As a result, we could not
> @@ -692,6 +696,30 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
> long adjust_next = 0;
> int remove_next = 0;
>
> + /*
> + * Why using vm_raw_write*() functions here to avoid lockdep's warning ?
> + *
> + * Locked is complaining about a theoretical lock dependency, involving
> + * 3 locks:
> + * mapping->i_mmap_rwsem --> vma->vm_sequence --> fs_reclaim
> + *
> + * Here are the major path leading to this dependency :
> + * 1. __vma_adjust() mmap_sem -> vm_sequence -> i_mmap_rwsem
> + * 2. move_vmap() mmap_sem -> vm_sequence -> fs_reclaim
> + * 3. __alloc_pages_nodemask() fs_reclaim -> i_mmap_rwsem
> + * 4. unmap_mapping_range() i_mmap_rwsem -> vm_sequence
> + *
> + * So there is no way to solve this easily, especially because in
> + * unmap_mapping_range() the i_mmap_rwsem is grab while the impacted
> + * VMAs are not yet known.
> + * However, the way the vm_seq is used is guarantying that we will
> + * never block on it since we just check for its value and never wait
> + * for it to move, see vma_has_changed() and handle_speculative_fault().
> + */
> + vm_raw_write_begin(vma);
> + if (next)
> + vm_raw_write_begin(next);
> +
> if (next && !insert) {
> struct vm_area_struct *exporter = NULL, *importer = NULL;
>

Eek, what about later on:

/*
* Easily overlooked: when mprotect shifts the boundary,
* make sure the expanding vma has anon_vma set if the
* shrinking vma had, to cover any anon pages imported.
*/
if (exporter && exporter->anon_vma && !importer->anon_vma) {
int error;

importer->anon_vma = exporter->anon_vma;
error = anon_vma_clone(importer, exporter);
if (error)
return error;
}

This needs

if (error) {
if (next && next != vma)
vm_raw_write_end(next);
vm_raw_write_end(vma);
return error;
}

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