Re: [PATCH] mm, virt: merge AS_UNMOVABLE and AS_INACCESSIBLE

[Michael Roth]

On Thu, Jul 11, 2024 at 02:03:05PM -0400, Paolo Bonzini wrote:
> The flags AS_UNMOVABLE and AS_INACCESSIBLE were both added just for guest_memfd;
> AS_UNMOVABLE is already in existing versions of Linux, while AS_INACCESSIBLE was
> acked for inclusion in 6.11.
> 
> But really, they are the same thing: only guest_memfd uses them, at least for
> now, and guest_memfd pages are unmovable because they should not be
> accessed by the CPU.
> 
> So merge them into one; use the AS_INACCESSIBLE name which is more comprehensive.
> At the same time, this fixes an embarrassing bug where AS_INACCESSIBLE was used
> as a bit mask, despite it being just a bit index.
> 
> The bug was mostly benign, becaus AS_INACCESSIBLE's bit representation (1010)
> corresponded to setting AS_UNEVICTABLE (which is already set) and AS_ENOSPC
> (except no async writes can happen on the guest_memfd).  So the AS_INACCESSIBLE
> flag simply had no effect.

*facepalm*. Thank you for catching this.

> 
> Fixes: 1d23040caa8b ("KVM: guest_memfd: Use AS_INACCESSIBLE when creating guest_memfd inode")
> Fixes: c72ceafbd12c ("mm: Introduce AS_INACCESSIBLE for encrypted/confidential memory")
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

I re-tested the AS_INACCESSIBLE handling with SNP. I also tested with
Sean's patch that enables THP support in gmem to test it since that was
the more problematic case.

Tested-by: Michael Roth <michael.roth@xxxxxxx>
Reviewed-by: Michael Roth <michael.roth@xxxxxxx>

> ---
>  include/linux/pagemap.h | 14 +++++++-------
>  mm/compaction.c         | 12 ++++++------
>  mm/migrate.c            |  2 +-
>  mm/truncate.c           |  2 +-
>  virt/kvm/guest_memfd.c  |  3 +--
>  5 files changed, 16 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index ce7bac8f81da..e05585eda771 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -208,8 +208,8 @@ enum mapping_flags {
>  	AS_RELEASE_ALWAYS,	/* Call ->release_folio(), even if no private data */
>  	AS_STABLE_WRITES,	/* must wait for writeback before modifying
>  				   folio contents */
> -	AS_UNMOVABLE,		/* The mapping cannot be moved, ever */
> -	AS_INACCESSIBLE,	/* Do not attempt direct R/W access to the mapping */
> +	AS_INACCESSIBLE,	/* Do not attempt direct R/W access to the mapping,
> +				   including to move the mapping */
>  };
>  
>  /**
> @@ -310,20 +310,20 @@ static inline void mapping_clear_stable_writes(struct address_space *mapping)
>  	clear_bit(AS_STABLE_WRITES, &mapping->flags);
>  }
>  
> -static inline void mapping_set_unmovable(struct address_space *mapping)
> +static inline void mapping_set_inaccessible(struct address_space *mapping)
>  {
>  	/*
> -	 * It's expected unmovable mappings are also unevictable. Compaction
> +	 * It's expected inaccessible mappings are also unevictable. Compaction
>  	 * migrate scanner (isolate_migratepages_block()) relies on this to
>  	 * reduce page locking.
>  	 */
>  	set_bit(AS_UNEVICTABLE, &mapping->flags);
> -	set_bit(AS_UNMOVABLE, &mapping->flags);
> +	set_bit(AS_INACCESSIBLE, &mapping->flags);
>  }
>  
> -static inline bool mapping_unmovable(struct address_space *mapping)
> +static inline bool mapping_inaccessible(struct address_space *mapping)
>  {
> -	return test_bit(AS_UNMOVABLE, &mapping->flags);
> +	return test_bit(AS_INACCESSIBLE, &mapping->flags);
>  }
>  
>  static inline gfp_t mapping_gfp_mask(struct address_space * mapping)
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e731d45befc7..714afd9c6df6 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1172,22 +1172,22 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  		if (((mode & ISOLATE_ASYNC_MIGRATE) && is_dirty) ||
>  		    (mapping && is_unevictable)) {
>  			bool migrate_dirty = true;
> -			bool is_unmovable;
> +			bool is_inaccessible;
>  
>  			/*
>  			 * Only folios without mappings or that have
>  			 * a ->migrate_folio callback are possible to migrate
>  			 * without blocking.
>  			 *
> -			 * Folios from unmovable mappings are not migratable.
> +			 * Folios from inaccessible mappings are not migratable.
>  			 *
>  			 * However, we can be racing with truncation, which can
>  			 * free the mapping that we need to check. Truncation
>  			 * holds the folio lock until after the folio is removed
>  			 * from the page so holding it ourselves is sufficient.
>  			 *
> -			 * To avoid locking the folio just to check unmovable,
> -			 * assume every unmovable folio is also unevictable,
> +			 * To avoid locking the folio just to check inaccessible,
> +			 * assume every inaccessible folio is also unevictable,
>  			 * which is a cheaper test.  If our assumption goes
>  			 * wrong, it's not a correctness bug, just potentially
>  			 * wasted cycles.
> @@ -1200,9 +1200,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  				migrate_dirty = !mapping ||
>  						mapping->a_ops->migrate_folio;
>  			}
> -			is_unmovable = mapping && mapping_unmovable(mapping);
> +			is_inaccessible = mapping && mapping_inaccessible(mapping);
>  			folio_unlock(folio);
> -			if (!migrate_dirty || is_unmovable)
> +			if (!migrate_dirty || is_inaccessible)
>  				goto isolate_fail_put;
>  		}
>  
> diff --git a/mm/migrate.c b/mm/migrate.c
> index dd04f578c19c..50b60fb414e9 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -965,7 +965,7 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
>  
>  		if (!mapping)
>  			rc = migrate_folio(mapping, dst, src, mode);
> -		else if (mapping_unmovable(mapping))
> +		else if (mapping_inaccessible(mapping))
>  			rc = -EOPNOTSUPP;
>  		else if (mapping->a_ops->migrate_folio)
>  			/*
> diff --git a/mm/truncate.c b/mm/truncate.c
> index 60388935086d..581977d2356f 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -233,7 +233,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
>  	 * doing a complex calculation here, and then doing the zeroing
>  	 * anyway if the page split fails.
>  	 */
> -	if (!(folio->mapping->flags & AS_INACCESSIBLE))
> +	if (!mapping_inaccessible(folio->mapping))
>  		folio_zero_range(folio, offset, length);
>  
>  	if (folio_has_private(folio))
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 9148b9679bb1..1c509c351261 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -416,11 +416,10 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
>  	inode->i_private = (void *)(unsigned long)flags;
>  	inode->i_op = &kvm_gmem_iops;
>  	inode->i_mapping->a_ops = &kvm_gmem_aops;
> -	inode->i_mapping->flags |= AS_INACCESSIBLE;
>  	inode->i_mode |= S_IFREG;
>  	inode->i_size = size;
>  	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
> -	mapping_set_unmovable(inode->i_mapping);
> +	mapping_set_inaccessible(inode->i_mapping);
>  	/* Unmovable mappings are supposed to be marked unevictable as well. */
>  	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
>  
> -- 
> 2.43.0
>