On Thu, Nov 16, 2023 at 04:00:40PM +1030, Qu Wenruo wrote:
> On 2023/11/16 15:35, Matthew Wilcox wrote:
> > On Thu, Nov 16, 2023 at 02:11:00PM +1030, Qu Wenruo wrote:
> > > E.g. if I allocated a folio with order 2, attached some private data to
> > > the folio, then call filemap_add_folio().
> > >
> > > Later some one called find_lock_page() and hit the 2nd page of that folio.
> > >
> > > I believe the regular IO is totally fine, but what would happen for the
> > > page->private of that folio?
> > > Would them all share the same value of the folio_attach_private()? Or
> > > some different values?
> >
> > Well, there's no magic ...
> >
> > If you call find_lock_page(), you get back the precise page. If you
> > call page_folio() on that page, you get back the folio that you stored.
> > If you then dereference folio->private, you get the pointer that you
> > passed to folio_attach_private().
> >
> > If you dereference page->private, *that is a bug*. You might get
> > NULL, you might get garbage. Just like dereferencing page->index or
> > page->mapping on tail pages. page_private() will also do the wrong thing
> > (we could fix that to embed a call to page_folio() ... it hasn't been
> > necessary before now, but if it'll help convert btrfs, then let's do it).
>
> That would be great. The biggest problem I'm hitting so far is the page
> cache for metadata.
>
> We're using __GFP_NOFAIL for the current per-page allocation, but IIRC
> __GFP_NOFAIL is ignored for higher order (>2 ?) folio allocation.
> And we may want that per-page allocation as the last resort effort
> allocation anyway.
>
> Thus I'm checking if there is something we can do here.
>
> But I guess we can always go folio_private() instead as a workaround for
> now?
I don't understand enough about what you're doing to offer useful
advice. Is this for bs>PS or is it arbitrary large folios for better
performance? If the latter, you can always fall back to order-0 folios.
If the former, well, we need to adjust a few things anyway to handle
filesystems with a minimum order ...
In general, you should be using folio_private(). page->private and
page_private() will be removed eventually.
The GFP_NOFAIL warning is:
WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
