Re: [RFC PATCH 2/2] KVM: guest_memfd: use filemap_grab_folios in write
From: Nikita Kalyazin
Date: Tue Jan 14 2025 - 11:09:36 EST
On 10/01/2025 21:08, Mike Day wrote:
On 1/10/25 09:46, Nikita Kalyazin wrote:
The write syscall on guest_memfd makes use of filemap_grab_folios to
grab folios in batches. This speeds up population by 8.3% due to the
reduction in locking and tree walking when adding folios to the
pagecache.
Signed-off-by: Nikita Kalyazin <kalyazin@xxxxxxxxxx>
---
virt/kvm/guest_memfd.c | 176 +++++++++++++++++++++++++++++++++--------
1 file changed, 143 insertions(+), 33 deletions(-)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index e80566ef56e9..ccfadc3a7389 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -102,17 +102,134 @@ static struct folio *kvm_gmem_get_folio(struct
inode *inode, pgoff_t index)
return filemap_grab_folio(inode->i_mapping, index);
}
+/*
+ * Returns locked folios on success. The caller is responsible for
+ * setting the up-to-date flag before the memory is mapped into the
guest.
+ * There is no backing storage for the memory, so the folios will remain
+ * up-to-date until they're removed.
+ *
+ * Ignore accessed, referenced, and dirty flags. The memory is
+ * unevictable and there is no storage to write back to.
+ */
+static int kvm_gmem_get_folios(struct inode *inode, pgoff_t index,
+ struct folio **folios, int num)
+{
+ return filemap_grab_folios(inode->i_mapping, index, folios, num);
+}
+
#if defined(CONFIG_KVM_GENERIC_PRIVATE_MEM) && !
defined(CONFIG_KVM_AMD_SEV)
+static int kvm_kmem_gmem_write_inner(struct inode *inode, pgoff_t index,
+ const void __user *buf,
+ struct folio **folios, int num)
+{
+ int ret, i, num_grabbed, num_written;
+
+ num_grabbed = kvm_gmem_get_folios(inode, index, folios, num);
+ if (num_grabbed < 0)
+ return num_grabbed;
+
+ for (i = 0; i < num_grabbed; i++) {
+ struct folio *folio = folios[i];
+ void *vaddr;
+
+ if (folio_test_hwpoison(folio)) {
+ folio_unlock(folio);
+ folio_put(folio);
+ ret = -EFAULT;
+ break;
+ }
+
+ if (folio_test_uptodate(folio)) {
+ folio_unlock(folio);
+ folio_put(folio);
+ ret = -ENOSPC;
+ break;
+ }
+
+ folio_unlock(folio);
+
+ vaddr = kmap_local_folio(folio, 0);
+ ret = copy_from_user(vaddr, buf + (i << PAGE_SHIFT),
PAGE_SIZE);
+ if (ret)
+ ret = -EINVAL;
+ kunmap_local(vaddr);
+
+ if (ret) {
+ folio_put(folio);
+ break;
+ } else {
+ kvm_gmem_mark_prepared(folio);
+ folio_put(folio);
+ }
+ }
+
+ num_written = i;
+
+ for (i = num_written; i < num_grabbed; i++) {
+ folio_unlock(folios[i]);
+ folio_put(folios[i]);
+ }
+
+ return num_written ?: ret;
+}
+
+static struct folio *kvm_kmem_gmem_write_folio(struct inode *inode,
pgoff_t index,
+ const char __user *buf)
This could probably be rewritten as:
struct folio *p_folio;
int ret;
ret = kvm_kmem_gmem_write_inner(inode, index, buf, &p_folio, 1);
if (ret == 1)
return p_folio;
else
return ERR_PTR(ret);
Would remove a few lines of duplicated code and use only one prototype.
Indeed! Thanks for the suggestion, will apply in the next revision.
Mike
+{
+ struct folio *folio;
+ void *vaddr;
+ int ret = 0;
+
+ folio = kvm_gmem_get_folio(inode, index);
+ if (IS_ERR(folio))
+ return ERR_PTR(-EFAULT);
+
+ if (folio_test_hwpoison(folio)) {
+ ret = -EFAULT;
+ goto out_unlock_put;
+ }
+
+ if (folio_test_uptodate(folio)) {
+ ret = -ENOSPC;
+ goto out_unlock_put;
+ }
+
+ folio_unlock(folio);
+
+ vaddr = kmap_local_folio(folio, 0);
+ ret = copy_from_user(vaddr, buf, PAGE_SIZE);
+ if (ret)
+ ret = -EINVAL;
+ kunmap_local(vaddr);
+
+ if (ret) {
+ folio_put(folio);
+ kvm_gmem_mark_prepared(folio);
+ goto out_err;
+ }
+
+ folio_put(folio);
+
+ return folio;
+
+out_unlock_put:
+ folio_unlock(folio);
+ folio_put(folio);
+out_err:
+ return ERR_PTR(ret);
+}
+
static ssize_t kvm_kmem_gmem_write(struct file *file, const char
__user *buf,
size_t count, loff_t *offset)
{
+ struct inode *inode = file_inode(file);
+ int ret = 0, batch_size = FILEMAP_GET_FOLIOS_BATCH_SIZE;
pgoff_t start, end, index;
- ssize_t ret = 0;
if (!PAGE_ALIGNED(*offset) || !PAGE_ALIGNED(count))
return -EINVAL;
- if (*offset + count > i_size_read(file_inode(file)))
+ if (*offset + count > i_size_read(inode))
return -EINVAL;
if (!buf)
@@ -123,9 +240,8 @@ static ssize_t kvm_kmem_gmem_write(struct file
*file, const char __user *buf,
filemap_invalidate_lock(file->f_mapping);
- for (index = start; index < end; ) {
- struct folio *folio;
- void *vaddr;
+ for (index = start; index + batch_size - 1 < end; ) {
+ struct folio *folios[FILEMAP_GET_FOLIOS_BATCH_SIZE] =
{ NULL };
pgoff_t buf_offset = (index - start) << PAGE_SHIFT;
if (signal_pending(current)) {
@@ -133,46 +249,40 @@ static ssize_t kvm_kmem_gmem_write(struct file
*file, const char __user *buf,
goto out;
}
- folio = kvm_gmem_get_folio(file_inode(file), index);
- if (IS_ERR(folio)) {
- ret = -EFAULT;
+ ret = kvm_kmem_gmem_write_inner(inode, index, buf +
buf_offset, folios, batch_size);
+ if (ret < 0)
goto out;
- }
- if (folio_test_hwpoison(folio)) {
- folio_unlock(folio);
- folio_put(folio);
- ret = -EFAULT;
+ index += ret;
+ if (ret < batch_size)
+ break;
+ }
+
+ for (; index < end; index++) {
+ struct folio *folio;
+ pgoff_t buf_offset = (index - start) << PAGE_SHIFT;
+
+ if (signal_pending(current)) {
+ ret = -EINTR;
goto out;
}
- if (folio_test_uptodate(folio)) {
- folio_unlock(folio);
- folio_put(folio);
- ret = -ENOSPC;
+ folio = kvm_kmem_gmem_write_folio(inode, index,
+ buf + buf_offset);
+ if (IS_ERR(folio)) {
+ ret = PTR_ERR(folio);
goto out;
}
-
- folio_unlock(folio);
-
- vaddr = kmap_local_folio(folio, 0);
- ret = copy_from_user(vaddr, buf + buf_offset, PAGE_SIZE);
- if (ret)
- ret = -EINVAL;
- kunmap_local(vaddr);
-
- kvm_gmem_mark_prepared(folio);
- folio_put(folio);
-
- index = folio_next_index(folio);
- *offset += PAGE_SIZE;
}
out:
filemap_invalidate_unlock(file->f_mapping);
+ if (index > start) {
+ *offset += (index - start) << PAGE_SHIFT;
+ return (index - start) << PAGE_SHIFT;
+ }
- return ret && start == (*offset >> PAGE_SHIFT) ?
- ret : *offset - (start << PAGE_SHIFT);
+ return ret;
}
#endif