Re: [PATCH v3 04/15] mm: update print_bad_page_map() to show anonymous page index

[Matthew Wilcox <willy@infradead.org>]

On Wed, Aug 05, 2026 at 10:01:43AM +0100, Lorenzo Stoakes (ARM) wrote:
> On Wed, Aug 05, 2026 at 04:30:18AM -0400, Gregory Price wrote:
> > Yeah just an F/A in the print I suppose.  Sorry if that was unclear.
> >
> > If it saves someone the head-scratch of whether some bug is because it's
> > "file" or "true anon", given the ambiguity here, seems like maybe useful
> > debug information.
> 
> Yeah will do on respin I think actually!
> 
> I do think not providing that could result in some confusion like 'hm I
> thought this was file-backed'.

not sure it would.  Look at the whole thing:

        pr_alert("BUG: Bad page map in process %s  %s:%08llx", current->comm,
                 pgtable_level_to_str(level), entry);
        __print_bad_page_map_pgtable(vma->vm_mm, addr);
        if (page)
                dump_page(page, "bad page map");
        pr_alert("addr:%px vm_flags:%08lx anon_vma:%px mapping:%px index:%lx\n",
                 (void *)addr, vma->vm_flags, vma->anon_vma, mapping, index);
        pr_alert("file:%pD fault:%ps mmap:%ps mmap_prepare: %ps read_folio:%ps\n",
                 vma->vm_file,
                 vma->vm_ops ? vma->vm_ops->fault : NULL,
                 vma->vm_file ? vma->vm_file->f_op->mmap : NULL,
                 vma->vm_file ? vma->vm_file->f_op->mmap_prepare : NULL,
                 mapping ? mapping->a_ops->read_folio : NULL);

even if we don't have 'page' and thus dump_page() won't tell us
anything, we look at mapping (NULL means true-anon, non-NULL means
file, possibly COW).

> (We've made our lives hard with varying definitions of 'anon'
> also... [shmem being 'anon in page cache' is especially lol] so it's
> already confusing but there we go :)

Woah, woah, woah.  Do *not* think of shmem as being anon-in-page-cache.
shmem is a filesystem that happens to use swap to store its backing
file.  I dislike a number of things about how it's implemented, but
describing it as anon-in-page-cache is furthering the confusion.