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

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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




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