On Mon, Jun 24, 2024 at 08:50:45PM GMT, Matthew Wilcox wrote: > On Mon, Jun 24, 2024 at 12:34:04PM -0700, Yosry Ahmed wrote: > > On Mon, Jun 24, 2024 at 12:26 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > On Mon, Jun 24, 2024 at 11:57:45AM -0700, Yosry Ahmed wrote: > > > > On Mon, Jun 24, 2024 at 11:56 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > > > > > On Mon, Jun 24, 2024 at 11:53:30AM -0700, Yosry Ahmed wrote: > > > > > > After a page is swapped out during reclaim, __remove_mapping() will > > > > > > call __delete_from_swap_cache() to replace the swap cache entry with a > > > > > > shadow entry (which is an xa_value). > > > > > > > > > > Special entries are disjoint from shadow entries. Shadow entries have > > > > > the last two bits as 01 or 11 (are congruent to 1 or 3 modulo 4). > > > > > Special entries have values below 4096 which end in 10 (are congruent > > > > > to 2 modulo 4). > > > > > > > > You are implying that we would no longer have a shadow entry for such > > > > zero folios, because we will be storing a special entry instead. > > > > Right? > > > > > > umm ... maybe I have a misunderstanding here. > > > > > > I'm saying that there wouldn't be a _swap_ entry here because the folio > > > wouldn't be stored anywhere on the swap device. But there could be a > > > _shadow_ entry. Although if the page is full of zeroes, it was probably > > > never referenced and doesn't really need a shadow entry. > > > > Is it possible to have a shadow entry AND a special entry (e.g. > > XA_ZERO_ENTRY) at the same index? This is what would be required to > > maintain the current behavior (assuming we really need the shadow > > entries for such zeroed folios). > > No, just like it's not possible to have a swap entry and a shadow entry > at the same location. You have to choose. But the zero entry is an > alternative to the swap entry, not the shadow entry. > > As I understand the swap cache, at the moment, you can have four > possible results from a lookup: > > - NULL > - a swap entry > - a shadow entry > - a folio > > Do I have that wrong? I don't think we have swap entry in the swapcache (underlying xarray). The swap entry is used as an index to find the folio or shadow entry.
