Re: [PATCH] Revert "mm, swap: avoid redundant swap device pinning"

[Kairui Song <ryncsn@gmail.com>]

On Mon, Nov 10, 2025 at 9:56 AM Huang, Ying
<ying.huang@linux.alibaba.com> wrote:
>
> Hi, Kairui,
>
> Kairui Song via B4 Relay <devnull+kasong.tencent.com@kernel.org> writes:
>
> > From: Kairui Song <kasong@tencent.com>
> >
> > This reverts commit 78524b05f1a3e16a5d00cc9c6259c41a9d6003ce.
> >
> > While reviewing recent leaf entry changes, I noticed that commit
> > 78524b05f1a3 ("mm, swap: avoid redundant swap device pinning") isn't
> > correct. It's true that most all callers of __read_swap_cache_async are
> > already holding a swap entry reference, so the repeated swap device
> > pinning isn't needed on the same swap device, but it is possible that
> > VMA readahead (swap_vma_readahead()) may encounter swap entries from a
> > different swap device when there are multiple swap devices, and call
> > __read_swap_cache_async without holding a reference to that swap device.
> >
> > So it is possible to cause a UAF if swapoff of device A raced with
> > swapin on device B, and VMA readahead tries to read swap entries from
> > device A. It's not easy to trigger but in theory possible to cause real
> > issues. And besides, that commit made swap more vulnerable to issues
> > like corrupted page tables.
> >
> > Just revert it. __read_swap_cache_async isn't that sensitive to
> > performance after all, as it's mostly used for SSD/HDD swap devices with
> > readahead. SYNCHRONOUS_IO devices may fallback onto it for swap count >
> > 1 entries, but very soon we will have a new helper and routine for
> > such devices, so they will never touch this helper or have redundant
> > swap device reference overhead.
>
> Is it better to add get_swap_device() in swap_vma_readahead()?  Whenever
> we get a swap entry, the first thing we need to do is call
> get_swap_device() to check the validity of the swap entry and prevent
> the backing swap device from going under us.  This helps us to avoid
> checking the validity of the swap entry in every swap function.  Does
> this sound reasonable?

Hi Ying, thanks for the suggestion!

Yes, that's also a feasible approach.

What I was thinking is that, currently except the readahead path, all
swapin entry goes through the get_swap_device() helper, that helper
also helps to mitigate swap entry corruption that may causes OOB or
NULL deref. Although I think it's really not that helpful at all to
mitigate page table corruption from the kernel side, but seems not a
really bad idea to have.

And the code is simpler this way, and seems more suitable for a stable
& mainline fix. If we want  to add get_swap_device() in
swap_vma_readahead(), we need to do that for every entry that doesn't
match the target entry's swap device. The reference overhead is
trivial compared to readhead and bio layer, and only non
SYNCHRONOUS_IO devices use this helper (madvise is a special case, we
may optimize that later). ZRAM may fallback to the readahead path but
this fallback will be eliminated very soon in swap table p2.

Another approach I thought about is that we might want readahead to
stop when it sees entries from a different swap device. That swap
device might be ZRAM where VMA readahead is not helpful.

How do you think?