On 03.02.22 14:12, Pedro Demarchi Gomes wrote:
> When "writes" is written to /sys/kernel/debug/damon/counter_type damon will monitor only writes.
> This patch also adds the actions mergeable and unmergeable to damos schemes. These actions are used by KSM as explained in [1].
[...]
>
> +static inline bool pte_is_pinned(struct vm_area_struct *vma, unsigned long addr, pte_t pte)
> +{
> + struct page *page;
> +
> + if (!pte_write(pte))
> + return false;
> + if (!is_cow_mapping(vma->vm_flags))
> + return false;
> + if (likely(!test_bit(MMF_HAS_PINNED, &vma->vm_mm->flags)))
> + return false;
> + page = vm_normal_page(vma, addr, pte);
> + if (!page)
> + return false;
> + return page_maybe_dma_pinned(page);
> +}
> +
> +static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
> + unsigned long addr, pmd_t *pmdp)
> +{
> + pmd_t old, pmd = *pmdp;
> +
> + if (pmd_present(pmd)) {
> + /* See comment in change_huge_pmd() */
> + old = pmdp_invalidate(vma, addr, pmdp);
> + if (pmd_dirty(old))
> + pmd = pmd_mkdirty(pmd);
> + if (pmd_young(old))
> + pmd = pmd_mkyoung(pmd);
> +
> + pmd = pmd_wrprotect(pmd);
> + pmd = pmd_clear_soft_dirty(pmd);
> +
> + set_pmd_at(vma->vm_mm, addr, pmdp, pmd);
> + } else if (is_migration_entry(pmd_to_swp_entry(pmd))) {
> + pmd = pmd_swp_clear_soft_dirty(pmd);
> + set_pmd_at(vma->vm_mm, addr, pmdp, pmd);
> + }
> +}
> +
> +static inline void clear_soft_dirty(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *pte)
> +{
> + /*
> + * The soft-dirty tracker uses #PF-s to catch writes
> + * to pages, so write-protect the pte as well. See the
> + * Documentation/admin-guide/mm/soft-dirty.rst for full description
> + * of how soft-dirty works.
> + */
> + pte_t ptent = *pte;
> +
> + if (pte_present(ptent)) {
> + pte_t old_pte;
> +
> + if (pte_is_pinned(vma, addr, ptent))
> + return;
> + old_pte = ptep_modify_prot_start(vma, addr, pte);
> + ptent = pte_wrprotect(old_pte);
> + ptent = pte_clear_soft_dirty(ptent);
> + ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
> + } else if (is_swap_pte(ptent)) {
> + ptent = pte_swp_clear_soft_dirty(ptent);
> + set_pte_at(vma->vm_mm, addr, pte, ptent);
> + }
> +}
Just like clearrefs, this can race against GUP-fast to detect pinned
pages. And just like clearrefs, we're not handling PMDs properly. And
just like anything that write-protects random anon pages right now, this
does not consider O_DIRECT as is.
Fortunately, there are not too many users of clearreefs/softdirty
tracking out there (my search a while ago returned no open source
users). My assumption is that your feature might see more widespread use.
Adding more random write protection until we fixed the COW issues [1]
really makes my stomach hurt on a Monday morning.
Please, let's defer any more features that rely on write-protecting
random anon pages until we have ways in place to not corrupt random user
space.
That is:
1) Teaching the COW logic to not copy pages that are pinned -- I'm
working on that.
2) Converting O_DIRECT to use FOLL_PIN instead of FOLL_GET. John is
working on that.
So I'm not against this change. I'm against this change at this point in
time.
[1]
https://lore.kernel.org/all/3ae33b08-d9ef-f846-56fb-645e3b9b4c66@xxxxxxxxxx/
--
Thanks,
David / dhildenb
