linux-kernel - Re: [PATCH v3 3/7] mm/swap: Add swp_offset

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Message-ID: <87bkssfxcf.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Wed, 10 Aug 2022 14:04:32 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Minchan Kim <minchan@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <andi.kleen@...el.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v3 3/7] mm/swap: Add swp_offset_pfn() to fetch PFN from
 swap entry

Peter Xu <peterx@...hat.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@...hat.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]