Re: [syzbot] [mm?] WARNING in __folio_rmap_sanity_checks (2)

David Hildenbrand
2025-01-10T16:35:30Z

On 31.12.24 09:41, Hillf Danton wrote:
> On Fri, 27 Dec 2024 20:56:21 -0800
>> syzbot has found a reproducer for the following issue on:
>>
>> HEAD commit: 8155b4ef3466 Add linux-next specific files for 20241220
>> git tree: linux-next
>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1652fadf980000
>
> #syz test
>
> --- x/mm/filemap.c
> +++ y/mm/filemap.c
> @@ -3636,6 +3636,10 @@ static vm_fault_t filemap_map_folio_rang
> continue;
> skip:
> if (count) {
> + for (unsigned int i = 0; i < count; i++) {
> + if (page_folio(page + i) != folio)
> + goto out;
> + }

IIRC, count <= nr_pages. Wouldn't that mean that we somehow pass in
nr_pages that already exceeds the given folio+start?

When I last looked at this, I was not able to spot the error in the
caller :(

> set_pte_range(vmf, folio, page, count, addr);
> *rss += count;
> folio_ref_add(folio, count);
> @@ -3658,6 +3662,7 @@ skip:
> ret = VM_FAULT_NOPAGE;
> }
>
> +out:
> vmf->pte = old_ptep;
>
> return ret;
> @@ -3702,7 +3707,7 @@ vm_fault_t filemap_map_pages(struct vm_f
> struct file *file = vma->vm_file;
> struct address_space *mapping = file->f_mapping;
> pgoff_t file_end, last_pgoff = start_pgoff;
> - unsigned long addr;
> + unsigned long addr, pmd_end;
> XA_STATE(xas, &mapping->i_pages, start_pgoff);
> struct folio *folio;
> vm_fault_t ret = 0;
> @@ -3731,6 +3736,12 @@ vm_fault_t filemap_map_pages(struct vm_f
> if (end_pgoff > file_end)
> end_pgoff = file_end;
>
> + /* make vmf->pte[x] valid */
> + pmd_end = ALIGN(addr, PMD_SIZE);
> + pmd_end = (pmd_end - addr) >> PAGE_SHIFT;
> + if (end_pgoff - start_pgoff > pmd_end)
> + end_pgoff = start_pgoff + pmd_end;
> +

do_fault_around() comments "This way it's easier to guarantee that we
don't cross page table boundaries."

It does some magic with PTRS_PER_PTE.

You're diff here seems to indicate that this is not the case?

But it's rather surprising that we see these issues pop up just now in
-next.

--
Cheers,

David / dhildenb