Re: [RFC 04/10] fs/dax: Don't track page mapping/index

Jan Kara
2024-04-12T15:22:12Z

On Thu 11-04-24 10:57:25, Alistair Popple wrote:
> The page->mapping and page->index fields are normally used by the
> pagecache and rmap for looking up virtual mappings of pages. FS DAX
> implements it's own kind of page cache and rmap look ups so these
> fields are unnecessary. They are currently only used to detect
> error/warning conditions which should never occur.
>
> A future change will change the way shared mappings are detected by
> doing normal page reference counting instead, so remove the
> unnecessary checks.
>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
...
> -/*
> - * When it is called in dax_insert_entry(), the shared flag will indicate that
> - * whether this entry is shared by multiple files. If so, set the page->mapping
> - * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount.
> - */
> -static void dax_associate_entry(void *entry, struct address_space *mapping,
> - struct vm_area_struct *vma, unsigned long address, bool shared)
> -{
> - unsigned long size = dax_entry_size(entry), pfn, index;
> - int i = 0;
> -
> - if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> - return;
> -
> - index = linear_page_index(vma, address & ~(size - 1));
> - for_each_mapped_pfn(entry, pfn) {
> - struct page *page = pfn_to_page(pfn);
> -
> - if (shared) {
> - dax_page_share_get(page);
> - } else {
> - WARN_ON_ONCE(page->mapping);
> - page->mapping = mapping;
> - page->index = index + i++;
> - }
> - }
> -}

Hum, but what about existing uses of folio->mapping and folio->index in
fs/dax.c? AFAICT this patch breaks them. What am I missing? How can this
ever work?

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR