Re: [RFC PATCH v2 0/6] KVM: gmem: Implement test cases for error_remove_page

[Paolo Bonzini <pbonzini@redhat.com>]

On 9/22/23 22:32, Sean Christopherson wrote:
> Unless we can't extend fadvise() for some reason, I think we should pursue
> FADV_HWPOISION.  The enabling should be downright trivial, e.g. just implement
> file_operations.fadvise() for guest_memfd, have it handle FADV_HWPOISON, and pass
> everything else to generic_fadvise().
> 
> It'll basically be your ioctl() just without a dedicated ioctl().
> 
> At the very least, we should run the idea past the fs maintainers.

fadvise() is different from madvise() though and not necessarily a great 
match.  Looking at the list of flags in advise(), something like 
FADV_POPULATE_READ, FADV_PAGEOUT or FADV_COLD would make sense, but I 
can't really think of any other flag that would be useful in a general 
case for fadvise.  Everything else would have to be very spcific to 
memfd or guest_memfd.

In particular FADV_HWPOISON would not make sense for anything that is 
not backend by memory.  There are some flags that could be useful on 
gmem file descriptors, such as hypothetically {WIPE,KEEP}ONFORK or 
SOFT_OFFLINE, but again they're not something that can be applied to 
fadvise().

So a ioctl implementation does have some advantages after all.  I 
suggest that we reuse MADV_* flags in the ioctl arguments, to leave the 
door open for future extensions and avoid ioctl proliferation.  The 
ioctl could be implemented by memfd, too, and perhaps even by /dev/zero.

Paolo