Re: [PATCH v12 18/45] KVM: guest_memfd: Handle lru_add fbatch refcounts during conversion safety check

Hugh Dickins [off-list ref] writes:

[...snip...]

quoted

@@ -926,6 +927,7 @@ void lru_cache_drain_for_folio(const struct folio *folio,
 			*drained = LRU_CACHE_DRAINED_ALL;
 	}
 }
+EXPORT_SYMBOL_FOR_KVM(lru_cache_drain_for_folio);

 atomic_t lru_disable_count = ATOMIC_INIT(0);

I don't mind about the virt/kvm/guest_memfd.c part of it, but I'm finding
a KVM patchset modifying mm/folio.c there hard to deal with: and notice
Sean also suggesting to separate this part out.

As you know, I've worked up a patchset "mm/fbatch: drain lru_add_drain()
and _all()" which finally removes the problem lru_cache_drain_for_folio()
works around. In the initial version posted a week ago, there was no
lru_cache_drain_for_folio() in the tree. Now 7.3-rc1 has it, so I
intended a replacement 13/25 in my series, giving you just an empty
inline lru_cache_drain_for_folio() stub (and enum lru_cache_drained)
in linux/swap.h.

But that won't work for you, if you're adding an EXPORT_SYMBOL_FOR_KVM()
in mm/folio.c, and of course conflicts with my removals (in context both
above and below your EXPORT line). It's easy for me to remove what's in
mm/gup.c and mm/folio.c, but I cannot remove what is not yet there.

I've wasted hours on this, hoping not to trouble either of you; but
seeing now that I shall have to rebase anyway (an unrelated mlock fix),
I'm electing to take the only clean way out: I'm going to submit this
mm/folio.c part of your patch to Andrew tonight (with a shorter Cc list!),
in the hope that it can be accelerated into 7.3-rc2 (or at least get an
mm-stable stable base-commit id) which we can both work off independently.

Thank you! Will use this in the next revision of this series.

Whether that's acceptable to Ackerley and to Andrew, I don't know
(just as we don't know when either of our patchsets will go further),
but let me try.

Thanks,
Hugh