Re: [PATCH v3 0/2] mm, swap: don't spin on a bad swap entry

[Andrew Morton]

On Tue, 18 Aug 2026 03:06:22 -0700 Breno Leitao <leitao@xxxxxxxxxx> wrote:

> I've seen some machines at Meta fleet that show the following type of
> problem:
> 
> 1) It gets some weird warning:
> 
>   BUG: Bad page map in process khugepaged  pte:f000eef300000017 pmd:00000067
>   addr:00007f57c0a01000 vm_flags:20200073 anon_vma:ffff88829af7c340 mapping:0000000000000000 index:7f57c0a01
> 
> The corruption is most likely the collapse/PT_RECLAIM race fixed by
> commit 366a4532d96f ("mm: fix the race between collapse and PT_RECLAIM
> under per-vma lock"). But this series is not about this one.
> 
> 2) Then the fault never makes progress. do_swap_page() returns 0 when
>    get_swap_device() fails, so the fault is retried, reads the same
>    entry and faults again. Nothing in the round trip changes the PTE,
>    and the same line comes out on every pass:
> 
>   get_swap_device: Bad swap offset entry 3ffffffc043c5
> 
> Patch 1 makes get_swap_device() return ERR_PTR(-EIO) for a malformed
> entry, keeping NULL for a device swapoff is taking away, and converts
> the callers. No functional change expected.
> 
> Patch 2 uses that to return VM_FAULT_SIGBUS instead of retrying.
> 
> The rate limiting patch that used to open this series was split out and
> posted on its own as a backportable hotfix [1], per Andrew's request. It
> should land first: patch 1 here touches the lines next to it in
> get_swap_device(). This patch will probably conflict with [1], but the
> merge should be trivial, given the only change in [1] is the
> addition of the __ratelimited() suffix. 
> 
> 	pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
> 	pr_err_ratelimited("%s: %s%08lx\n", __func__, Bad_file, entry.val);
> 

Thanks.  The patches have bitrotted a little - get_swap_device() was an
easy fixup but please double-check that I didn't miss anything.

Or perhaps just refresh-retest-resend if there's any doubt.


> The rate limiting patch that used to open this series was split out and
> posted on its own as a backportable hotfix [1], per Andrew's request. It
> should land first: patch 1 here touches the lines next to it in
> get_swap_device(). This patch will probably conflict with [1], but the
> merge should be trivial, given the only change in [1] is the
> addition of the __ratelimited() suffix. 
> 
> 	pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
> 		pr_err_ratelimited("%s: %s%08lx\n", __func__, Bad_file, entry.val);

oh, you already said that.

As usual when inspecting our error-path code, Sashiko said "you all suck":

	https://sashiko.dev/#/patchset/20260818-swap-v3-0-d3fa52598a59@xxxxxxxxxx

These things do seem on-topic for the changes you're proposing here, so
please take a look?