Re: [RFC][PATCH] uprobe: support for private hugetlb mappings

David Hildenbrand
2024-04-30T18:21:16Z

On 30.04.24 17:22, Guillaume Morin wrote:
> On 26 Apr 21:55, Guillaume Morin wrote:
>>
>> On 26 Apr 9:19, David Hildenbrand wrote:
>>> A couple of points:
>>>
>>> a) Don't use page_mapcount(). Either folio_mapcount(), but likely you want
>>> to check PageAnonExclusive.
>>>
>>> b) If you're not following the can_follow_write_pte/_pmd model, you are
>>> doing something wrong :)
>>>
>>> c) The code was heavily changed in mm/mm-unstable. It was merged with t
>>> the common code.
>>>
>>> Likely, in mm/mm-unstable, the existing can_follow_write_pte and
>>> can_follow_write_pmd checks will already cover what you want in most cases.
>>>
>>> We'd need a can_follow_write_pud() to cover follow_huge_pud() and
>>> (unfortunately) something to handle follow_hugepd() as well similarly.
>>>
>>> Copy-pasting what we do in can_follow_write_pte() and adjusting for
>>> different PTE types is the right thing to do. Maybe now it's time to factor
>>> out the common checks into a separate helper.
>>
>> I tried to get the hugepd stuff right but this was the first I heard
>> about it :-) Afaict follow_huge_pmd and friends were already DTRT
>
> I got it working on top of your uprobes-cow branch with the foll force
> patch sent friday. Still pretty lightly tested

Sorry for not replying earlier, was busy with other stuff. I'll try
getiing that stuff into shape and send it out soonish.

>
> I went with using one write uprobe function with some additional
> branches. I went back and forth between that and making them 2 different
> functions.

All the folio_test_hugetlb() special casing is a bit suboptimal. Likely
we want a separate variant, because we should be sing hugetlb PTE
functions consistently (e.g., huge_pte_uffd_wp() vs pte_uffd_wp(),
softdirty does not exist etc.)

>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 2f4e88552d3f..8a33e380f7ea 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -83,6 +83,10 @@ static const struct fs_parameter_spec hugetlb_fs_parameters[] = {
> {}
> };
>
> +bool hugetlbfs_mapping(struct address_space *mapping) {
> + return mapping->a_ops == &hugetlbfs_aops;

is_vm_hugetlb_page() might be what you are looking for.

[...]

> }
>
> -static void copy_from_page(struct page *page, unsigned long vaddr, void *dst, int len)
> +static void copy_from_page(struct page *page, unsigned long vaddr, void *dst, int len, unsigned long page_mask)
> {
> void *kaddr = kmap_atomic(page);
> - memcpy(dst, kaddr + (vaddr & ~PAGE_MASK), len);
> + memcpy(dst, kaddr + (vaddr & ~page_mask), len);
> kunmap_atomic(kaddr);
> }

>
> -static void copy_to_page(struct page *page, unsigned long vaddr, const void *src, int len)
> +static void copy_to_page(struct page *page, unsigned long vaddr, const void *src, int len, unsigned long page_mask)
> {
> void *kaddr = kmap_atomic(page);
> - memcpy(kaddr + (vaddr & ~PAGE_MASK), src, len);
> + memcpy(kaddr + (vaddr & ~page_mask), src, len);
> kunmap_atomic(kaddr);
> }

These two changes really are rather ugly ...

An why are they even required? We get a PAGE_SIZED-based subpage of a
hugetlb page. We only kmap that one and copy within that one.

In other words, I don't think the copy_from_page() and copy_to_page()
changes are even required when we consistently work on subpages and not
suddenly on head pages.

>
> -static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t *new_opcode)
> +static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t *new_opcode, unsigned long page_mask)
> {
> uprobe_opcode_t old_opcode;
> bool is_swbp;
> @@ -191,7 +192,8 @@ static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t
> * is a trap variant; uprobes always wins over any other (gdb)
> * breakpoint.
> */
> - copy_from_page(page, vaddr, &old_opcode, UPROBE_SWBP_INSN_SIZE);
> + copy_from_page(page, vaddr, &old_opcode, UPROBE_SWBP_INSN_SIZE,
> + page_mask);
> is_swbp = is_swbp_insn(&old_opcode);
>
> if (is_swbp_insn(new_opcode)) {
> @@ -376,8 +378,8 @@ struct uwo_data {
> uprobe_opcode_t opcode;
> };
>
> -static int __write_opcode_pte(pte_t *ptep, unsigned long vaddr,
> - unsigned long next, struct mm_walk *walk)
> +static int __write_opcode(pte_t *ptep, unsigned long vaddr,
> + unsigned long page_mask, struct mm_walk *walk)

Unrelated alignment change.

> {
> struct uwo_data *data = walk->private;;
> const bool is_register = !!is_swbp_insn(&data->opcode);
> @@ -415,9 +417,12 @@ static int __write_opcode_pte(pte_t *ptep, unsigned long vaddr,
>
> /* Unmap + flush the TLB, such that we can write atomically .*/
> flush_cache_page(vma, vaddr, pte_pfn(pte));
> - pte = ptep_clear_flush(vma, vaddr, ptep);
> + if (folio_test_hugetlb(folio))
> + pte = huge_ptep_clear_flush(vma, vaddr, ptep);
> + else
> + pte = ptep_clear_flush(vma, vaddr, ptep);
> copy_to_page(page, data->opcode_vaddr, &data->opcode,
> - UPROBE_SWBP_INSN_SIZE);
> + UPROBE_SWBP_INSN_SIZE, page_mask);
>
> /* When unregistering, we may only zap a PTE if uffd is disabled ... */
> if (is_register || userfaultfd_missing(vma))
> @@ -443,13 +448,18 @@ static int __write_opcode_pte(pte_t *ptep, unsigned long vaddr,
> if (!identical || folio_maybe_dma_pinned(folio))
> goto remap;
>
> - /* Zap it and try to reclaim swap space. */
> - dec_mm_counter(mm, MM_ANONPAGES);
> - folio_remove_rmap_pte(folio, page, vma);
> - if (!folio_mapped(folio) && folio_test_swapcache(folio) &&
> - folio_trylock(folio)) {
> - folio_free_swap(folio);
> - folio_unlock(folio);
> + if (folio_test_hugetlb(folio)) {
> + hugetlb_remove_rmap(folio);
> + large = false;
> + } else {
> + /* Zap it and try to reclaim swap space. */
> + dec_mm_counter(mm, MM_ANONPAGES);
> + folio_remove_rmap_pte(folio, page, vma);
> + if (!folio_mapped(folio) && folio_test_swapcache(folio) &&
> + folio_trylock(folio)) {
> + folio_free_swap(folio);
> + folio_unlock(folio);
> + }
> }
> folio_put(folio);
>
> @@ -461,11 +471,29 @@ static int __write_opcode_pte(pte_t *ptep, unsigned long vaddr,
> */
> smp_wmb();
> /* We modified the page. Make sure to mark the PTE dirty. */
> - set_pte_at(mm, vaddr, ptep, pte_mkdirty(pte));
> + if (folio_test_hugetlb(folio))
> + set_huge_pte_at(mm , vaddr, ptep, huge_pte_mkdirty(pte),
> + (~page_mask) + 1);
> + else
> + set_pte_at(mm, vaddr, ptep, pte_mkdirty(pte));
> return UWO_DONE;
> }
>
> +static int __write_opcode_hugetlb(pte_t *ptep, unsigned long hmask,
> + unsigned long vaddr,
> + unsigned long next, struct mm_walk *walk)
> +{
> + return __write_opcode(ptep, vaddr, hmask, walk);
> +}
> +
> +static int __write_opcode_pte(pte_t *ptep, unsigned long vaddr,
> + unsigned long next, struct mm_walk *walk)
> +{
> + return __write_opcode(ptep, vaddr, PAGE_MASK, walk);
> +}
> +
> static const struct mm_walk_ops write_opcode_ops = {
> + .hugetlb_entry = __write_opcode_hugetlb,
> .pte_entry = __write_opcode_pte,
> .walk_lock = PGWALK_WRLOCK,
> };
> @@ -492,7 +520,7 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> unsigned long opcode_vaddr, uprobe_opcode_t opcode)
> {
> struct uprobe *uprobe = container_of(auprobe, struct uprobe, arch);
> - const unsigned long vaddr = opcode_vaddr & PAGE_MASK;
> + unsigned long vaddr = opcode_vaddr & PAGE_MASK;
> const bool is_register = !!is_swbp_insn(&opcode);
> struct uwo_data data = {
> .opcode = opcode,
> @@ -503,6 +531,7 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> struct mmu_notifier_range range;
> int ret, ref_ctr_updated = 0;
> struct page *page;
> + unsigned long page_size = PAGE_SIZE;
>
> if (WARN_ON_ONCE(!is_cow_mapping(vma->vm_flags)))
> return -EINVAL;
> @@ -521,7 +550,14 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct vm_area_struct *vma,
> if (ret != 1)
> goto out;
>
> - ret = verify_opcode(page, opcode_vaddr, &opcode);
> +
> + if (is_vm_hugetlb_page(vma)) {
> + struct hstate *h = hstate_vma(vma);
> + page_size = huge_page_size(h);
> + vaddr &= huge_page_mask(h);
> + page = compound_head(page);

I think we should only adjust the range we pass to the mmu notifier and
for walking the VMA range. But we should not adjust vaddr.

Further, we should not adjust the page if possible ... ideally, we'll
treat hugetlb folios just like large folios here and operate on subpages.

Inside __write_opcode(), we can derive the the page of interest from
data->opcode_vaddr.

find_get_page() might need some though, if it won't return a subpage of
a hugetlb folio. Should be solvable by a wrapper, though.

> + }
> + ret = verify_opcode(page, opcode_vaddr, &opcode, ~(page_size - 1));
> put_page(page);
> if (ret <= 0)
> goto out;

--
Cheers,

David / dhildenb