Re: [PATCH v6 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

[David Hildenbrand]

On 08.04.24 12:07, Ryan Roberts wrote:
> On 08/04/2024 10:43, David Hildenbrand wrote:
> > > > > +
> > > > > +/**
> > > > > + * swap_pte_batch - detect a PTE batch for a set of contiguous swap entries
> > > > > + * @start_ptep: Page table pointer for the first entry.
> > > > > + * @max_nr: The maximum number of table entries to consider.
> > > > > + * @entry: Swap entry recovered from the first table entry.
> > > > > + *
> > > > > + * Detect a batch of contiguous swap entries: consecutive (non-present) PTEs
> > > > > + * containing swap entries all with consecutive offsets and targeting the same
> > > > > + * swap type.
> > > > > + *
> > > >
> > > > Likely you should document that any swp pte bits are ignored? ()
> > >
> > > Sorry I don't understand this comment. I thought any non-none, non-present PTE
> > > was always considered to contain only a "swap entry" and a swap entry consists
> > > of a "type" and an "offset" only. (and its a special "non-swap" swap entry if
> > > type > SOME_CONSTANT) Are you saying there are additional fields in the PTE that
> > > are not part of the swap entry?
> >
> >
> > pte_swp_soft_dirty()
> > pte_swp_clear_exclusive()
> > pte_swp_uffd_wp()
> >
> > Are PTE bits used for swp PTE.
>
> Ahh wow. mind blown. Looks like a massive hack... why not store them in the
> arch-independent swap entry, rather than have them squat independently in the PTE?

I think that was discussed at some point, but it not only requires quite 
some churn to change it (all that swp entry code is a mess), these bits 
are conceptually really per PTE and not something you would want to pass 
into actual swap handling code that couldn't care less about all of these.

I looked at this when I added SWP exclusive, but accidentally losing the 
SWP exclusive marker when converting back and forth made me go the PTE 
route instead.

Then, the available PTE bits are a bit scattered on some architectures, 
and converting entry<->PTE gets even uglier if we don't want to "lose" 
available bits.

Probably the whole "unsigned long swp_entry" stuff should be replaced by 
a proper struct where we could more easily add flags and have the arch 
code handle the conversion to the PTE just once. The arch-specific 
swp_entry stuff is another nasty thing IMHO.

> OK, my implementation is buggy. I'll re-spin to fix this.
>
>
> > There is also dirty/young for migration entries, but that's not of a concern
> > here, because we stop for non_swap_entry().
>
> Looks like these are part of the offset field in the arch-independent swap entry
> - much cleaner ;-).

Note that it only applies to migration entries, and only when we have 
some spare bits due to PFN < offset.

--
Cheers,

David / dhildenb