On Sun, 30 Aug 2026, Ackerley Tng via B4 Relay wrote: > From: Ackerley Tng <[email protected]> > > A guest_memfd folio has no outstanding references if guest_memfd holds the > only references on it. Any other references on the folio may indicate > another user, and guest_memfd cannot convert it to private if there may be > an existing host user. > > A folio will have outstanding references if it is present in a per-CPU > lru_add fbatch. guest_memfd does not actually participate in LRU, but > freshly-allocated folios are still added to the lru_add fbatch for batch > LRU statistics processing. > > A folio may also have extra refcounts if it is on the mlock fbatch. > > These two known "usages" of the folio are handled by calling > lru_cache_drain_for_folio, which drains both the lru_add and mlock > fbatches. After draining, if the refcount is still elevated, then there are > truly outstanding references. > > If the page may be dma pinned, DMA is using it and hence there are > outstanding references. folio_maybe_dma_pinned() can have false positives, > but that's only with a significant number of refcounts, at which point > draining LRU is not going to move the needle - it can still be concluded > that the folio has outstanding references. > > If the page is still mapped after guest_memfd tried to unmap it earlier in > the conversion process, it also has outstanding references. > > Return true and exit early to avoid unnecessary draining in these 2 cases. > > Provide a drain status to only drain once ever while processing a batch of > folios. > > Acked-by: Vlastimil Babka (SUSE) <[email protected]> > Suggested-by: David Hildenbrand <[email protected]> > Reviewed-by: Fuad Tabba <[email protected]> > Reviewed-by: Binbin Wu <[email protected]> > Signed-off-by: Ackerley Tng <[email protected]> > --- > mm/folio.c | 2 ++ > virt/kvm/guest_memfd.c | 30 ++++++++++++++++++++++-------- > 2 files changed, 24 insertions(+), 8 deletions(-) > > diff --git a/mm/folio.c b/mm/folio.c > index c02dcea9c03c2..50a6dbe55998e 100644 > --- a/mm/folio.c > +++ b/mm/folio.c > @@ -33,6 +33,7 @@ > #include <linux/page_idle.h> > #include <linux/local_lock.h> > #include <linux/buffer_head.h> > +#include <linux/kvm_types.h> > > #include "internal.h" > #include "page_alloc.h" > @@ -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. 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
