Re: [PATCH v3 3/7] mm/swap: Add swp_offset_pfn() to fetch PFN from swap entry

["Huang, Ying" <ying.huang@intel.com>]

Peter Xu <peterx@redhat.com> writes:

> We've got a bunch of special swap entries that stores PFN inside the swap
> offset fields.  To fetch the PFN, normally the user just calls swp_offset()
> assuming that'll be the PFN.
>
> Add a helper swp_offset_pfn() to fetch the PFN instead, fetching only the
> max possible length of a PFN on the host, meanwhile doing proper check with
> MAX_PHYSMEM_BITS to make sure the swap offsets can actually store the PFNs
> properly always using the BUILD_BUG_ON() in is_pfn_swap_entry().
>
> One reason to do so is we never tried to sanitize whether swap offset can
> really fit for storing PFN.  At the meantime, this patch also prepares us
> with the future possibility to store more information inside the swp offset
> field, so assuming "swp_offset(entry)" to be the PFN will not stand any
> more very soon.
>
> Replace many of the swp_offset() callers to use swp_offset_pfn() where
> proper.  Note that many of the existing users are not candidates for the
> replacement, e.g.:
>
>   (1) When the swap entry is not a pfn swap entry at all, or,
>   (2) when we wanna keep the whole swp_offset but only change the swp type.
>
> For the latter, it can happen when fork() triggered on a write-migration
> swap entry pte, we may want to only change the migration type from
> write->read but keep the rest, so it's not "fetching PFN" but "changing
> swap type only".  They're left aside so that when there're more information
> within the swp offset they'll be carried over naturally in those cases.
>
> Since at it, dropping hwpoison_entry_to_pfn() because that's exactly what
> the new swp_offset_pfn() is about.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>

The patch itself looks good.  But I searched swp_entry() in kernel
source code, and found that we need to do more.

For example, in pte_to_pagemap_entry()

			frame = swp_type(entry) |
				(swp_offset(entry) << MAX_SWAPFILES_SHIFT);

If it's a migration entry, we need

			frame = swp_type(entry) |
				(swp_offset_pfn(entry) << MAX_SWAPFILES_SHIFT);

So I think you need to search all swp_offset() calling in the kernel
source and check whether they need to be changed.

Best Regards,
Huang, Ying

[snip]