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Message-ID: <7cf9f0fd-279e-4e4d-99ac-966a752090a2@amd.com>
Date: Fri, 10 Jan 2025 15:08:08 -0600
From: Mike Day <michael.day@....com>
To: Nikita Kalyazin <kalyazin@...zon.com>, willy@...radead.org,
pbonzini@...hat.com, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc: david@...hat.com, jthoughton@...gle.com, michael.roth@....com,
ackerleytng@...gle.com, graf@...zon.de, jgowans@...zon.com,
roypat@...zon.co.uk, derekmn@...zon.com, nsaenz@...zon.es,
xmarcalx@...zon.com
Subject: Re: [RFC PATCH 2/2] KVM: guest_memfd: use filemap_grab_folios in
write
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@...zon.com>
> ---
> 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.
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
>
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