Re: [PATCH] vfio/type1: Retry follow_pfnmap_start() when PFNMAP is zapped

["Boone, Max" <mboone@akamai.com>]

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> On Mar 18, 2026, at 10:22 PM, Alex Williamson <alex@shazbot.org> wrote:
> 
> […]
> 
>> + /*
>> + * follow_pfnmap_start() returns -EINVAL for
>> + * invalid parameters and non-present entries.
>> + * If that happens here after a successful
>> + * fixup_user_fault(), it is likely that the
>> + * pfnmap has been zapped. Retry instead of
>> + * failing.
>> + */
> 
> It's a little stronger than that, right?  We're betting that the only
> remaining non-zero return is due to a race and we can introduce what
> appears to be potential for an infinite loop here because -EAGAIN will
> get kicked out to redo the vma_lookup() and fixup_user_fault() should
> return a genuine error if we're completely in the weeds.  Should we
> make this a little stronger and more specific?  Thanks,

I’d say that the best case would be to have follow_pfnmap_start() return
-EINVAL or -ENOENT w.r.t. which of the two return values it is. But then
again, we could theoretically run into an infinite loop I guess - as the zap
and faulting could run in lockstep (the race window is extremely small
though).

We could make the retry above bounded, and bubble up a -EBUSY such
that users of the ioctl can decide to retry instead of fail?

David, you mentioned that gup already has retry logic that we don’t have
with follow_fault_pfn() -> follow_pfnmap_start(). Would we potentially run
into an infinite loop with this change?

I see that the same pattern also appears in:
- virt/kvm/kvm_main.c:hva_to_pfn_remapped()
- s390/pci/pci_mmio.c:s390_pci_mmio_write()

And other users of the function are:
- drivers/virt/acrn/mm.c:acrn_vm_ram_map()
- mm/memory.c:generic_access_phys()

Maybe I can better draft in this patch with adding an -ENOENT return to
follow_pfnmap_start() and in the same patchset add retrying logic where
necessary (on first sight, kvm and vfio)?

> 
> Alex
> 
>> ret = follow_pfnmap_start(&args);
>> if (ret)
>> - return ret;
>> + return -EAGAIN;
>> }
>> 
>> if (write_fault && !args.writable) {
>> 
>> ---
>> base-commit: 96ca4caf9066f5ebd35b561a521af588a8eb0215
>> change-id: 20260317-retry-pin-on-reclaimed-pud-dfb9e26eb8cf
>> 
>> Best regards,
> 


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