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

Dan Williams
2024-04-12T17:31:59Z

Jan Kara wrote:
> 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?

Right, as far as I can see every fsdax filesystem would need to be
converted to use dax_holder_operations() so that the fs can backfill
->mapping and ->index.