[RFC PATCH v1 07/10] KVM: guest_memfd: Implement custom truncation function

[Ackerley Tng]

Implement custom truncation function for guest_memfd, and replace calls to
truncate_inode_pages_range() with calls to this custom truncation function.

The custom truncation function removes a lot of the generality supported by
truncate_inode_pages_range() not required by guest_memfd, such as

+ sub-PAGE_SIZE truncations
+ Support for writeback

In a later patch, guest_memfd use this custom truncation function to handle
updating of i_blocks and i_bytes in the inode during truncation.

Signed-off-by: Ackerley Tng <ackerley...@google.com>
---
 virt/kvm/guest_memfd.c | 43 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 57dec458bfa77..e6c66ab7062b3 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -247,6 +247,45 @@ static void kvm_gmem_invalidate_end(struct inode *inode, 
pgoff_t start,
                __kvm_gmem_invalidate_end(f, start, end);
 }
 
+static void kvm_gmem_truncate_folio(struct folio *folio)
+{
+       folio_lock(folio);
+
+       if (folio_mapped(folio))
+               unmap_mapping_folio(folio);
+
+       /*
+        * guest_memfd doesn't need writeback, skip anything to do with
+        * writeback and just clear the dirty flag.
+        */
+       folio_clear_dirty(folio);
+       filemap_remove_folio(folio);
+
+       folio_unlock(folio);
+}
+
+static void kvm_gmem_truncate_range(struct inode *inode, pgoff_t start,
+                                   size_t nr_pages)
+
+{
+       struct folio_batch fbatch;
+       pgoff_t next;
+       pgoff_t last;
+       int i;
+
+       last = start + nr_pages - 1;
+
+       folio_batch_init(&fbatch);
+       next = start;
+       while (filemap_get_folios(inode->i_mapping, &next, last, &fbatch)) {
+               for (i = 0; i < folio_batch_count(&fbatch); ++i)
+                       kvm_gmem_truncate_folio(fbatch.folios[i]);
+
+               folio_batch_release(&fbatch);
+               cond_resched();
+       }
+}
+
 static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 {
        pgoff_t start = offset >> PAGE_SHIFT;
@@ -260,7 +299,7 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t 
offset, loff_t len)
 
        kvm_gmem_invalidate_begin(inode, start, end);
 
-       truncate_inode_pages_range(inode->i_mapping, offset, offset + len - 1);
+       kvm_gmem_truncate_range(inode, offset, len >> PAGE_SHIFT);
 
        kvm_gmem_invalidate_end(inode, start, end);
 
@@ -984,7 +1023,7 @@ static void kvm_gmem_evict_inode(struct inode *inode)
 
        truncate_inode_pages_final_prepare(mapping);
 
-       truncate_inode_pages_range(mapping, 0, inode->i_size);
+       kvm_gmem_truncate_range(inode, 0, inode->i_size >> PAGE_SHIFT);
 
        clear_inode(inode);
 }
-- 
2.53.0.345.g96ddfc5eaa-goog